Clearing up the misinformation on winter tires - from an automotive professional
(Remaking this as a post after I wrote another dissertation of a comment on someone else's post)
Ok I'm going to do this again because every time this comes up I see so much misinformation. I'm a mechanic, I've run several different automotive businesses, even a tire shop for a few years.
First, they aren't snow tires. They're winter tires. They're called this because there's huge traction benefits with these tires in all winter conditions, not just snow and ice. They help when it's below freezing outside because when the temps drop the rubber in most non-winter tires hardens and when it's really cold if you don't have winter tires on then you're pretty much driving on tires that are frozen and now might as well be made of stone. Second, more than 99% of you are using all-season tires as your summer or year-round set, not summer tires. "Summer" tires are pretty much racetrack tires, they're rock hard and grip the shit out of dry pavement but have virtually no traction in even the lightest of rains and they're undrivable in moderate to heavy rain or any snow at all. Every person who isn't driving a fast, performance car extra aggressively or running a car on a track can forget about summer tires. More on all-seasons later.
Now with winter tires their primary purpose is to provide improved winter traction. That means not just in snow and ice but in cold weather too. The main ways they're able to provide the improved traction is because of the composition of the rubber. It's made softer and more porous to remain flexible where other rubber compounds harden in sub-freezing temps and to help wick away the thin layer of water that forms between the tire and the road surface. Slippage happens much worse when there's packed snow or ice on the road, you aren't really slipping on the ice you're slipping on the boundary layer of water that forms instantaneously between the weight of the tire and the ice - ice itself isn't really inherently slippery. Winter compound tires are much softer and more porous than all-season tires. And because of this they aren't supposed to be used during warmer months because of the hot pavement you end up driving them on. On hot days and during highway driving the tires actually build up tons of heat and it's enough to bake the winter compound rubber and harden it to the point it eliminates most of the properties of the rubber that make it effective as a winter tire. And by running around all summer on them now over time you're baking them into something that more closely resembles an all-season tire rubber compound. This is not even to mention the rate at which the winter rubber naturally wears away which is obviously higher than other tire options as well which of course means less miles on the tires overall even when not run on hot roads. Good winter tires will work well for their intended purposes for 15k-25k miles depending on how and where they're driven.
On all-season tires, you really have to keep in mind that it was not the best choice of names that was chosen for this category. A more appropriate name would be "totally acceptable for 3 seasons and not completely deadly like a 'summer' tire is in the winter" tires but "all-season" is a lot more convenient. All-season tires are almost always specified by the manufacturer to not be recommended for below freezing temperatures. All-season tires are typically designed to be the best compromise a manufacturer can find between treadwear (longevity) and traction (grip) in the most types of driving and road conditions. If a tire could last 100k miles but skates all over the road if there's a drop of water out there then the treadwear is great but the traction sucks. If a tire performs exceptionally well in the winter and blasts through snow and ice like the roads are bare like some winters do then the tradeoff is they have less useable miles in them from the start. With these the traction is great and the treadwear is reduced as a tradeoff. Good all-season tires regularly last 30-60k or even a lot more in some cases but there's a lot of variables.
There are also categories of tires along with winter and all-season that are called all-terrain tires and the newest kind that's been gaining rapid popularity is all-weather tires. All-terrain is the more off-road kind of tire most people are used to seeing on pickup trucks and SUVs and such. You can get them in any size though, even for your Subaru. They're usually great for traction, not very good for treadwear, and they tend to be noisy at higher speeds because of the air moving through the large gaps in the tread blocks. All-weather tires are a great option for lots of people around here, they're essentially a hybrid of a winter tire and an all-season tire. People in climates like ours obviously can see lots of benefits from not needing to own two sets of tires and the improvements in tire technology over the last 10-15 years or so has seen this category ballooning in usage, options, and performance. With an all-weather tire you get better wet and winter traction than an all-season, but not as good as a winter. You get better treadwear than a winter, but not as good as an all-season. The winter/snow/ice performance of an all-weather tire also takes a nosedive in the second half of their useable life and really just performs like an all-season tire that wears out more quickly than it otherwise would. They're a great choice for a lot of people around here but I don't recommend them to everyone. If you do a lot of highway driving in the summer or put on quite a lot of miles then they aren't usually a good choice. Good all-weather or all-terrain tires typically last in the 20-40k range but 25k-ish is the most common and these categories continue to see the most noticeable improvements as time progresses.
The best indicator of a tire's ability to help you out and perform better in snow, ice, and below freezing temperatures is if the tire has the 3-Peak Mountain Snowflake (3PMSF) designation. This means they've been proven to perform in snow/ice/cold at a certain level that's deemed acceptable to earn the designation. All winter tires, all all-weather tires, and some all-terrain tires have the 3PMSF designation and these are the ones that do great during our winters here. A lot of all-terrain tires are not softer rubber and are not at all good on slippery roads and they do not have the 3PMSF designation. The M+S "mud and snow" designation that shows up on most all tires is something you can generally just ignore. Almost every road tire out there has this designation and it just means that a certain portion of the tread area is negative space and not tread block so virtually all modern tires fit the description to earn this designation. It doesn't actually mean they're worth a shit in mud or snow. Any manufacturer can put that on a tire just based on the look of the tread pattern so it really means nothing. There's actual testing and performance metrics a tire must meet to earn the 3PMSF tag and if it's got this then it's almost always going to be something you'll see dramatic improvements driving in crappy conditions. Tons and tons of people think their all-season M+S tires grip just as good as a true winter tire does but they're just plain wrong.
All of these reasons are why there are different categories of tires and why for most people it's recommended but obviously not required to have two sets. The average person usually does see the return on investment from running one winter set (better traction, you and arguably more importantly everyone else on the road is safer, one time bouncing your car off a guard rail almost always costs more in repairs than a set of tires does, etc) and one all-season set (much better treadwear, less road noise, better stopping distances on dry and/or hot pavement etc.)
There are always going to be the "I always run all-seasons and I've always been fine" or "winter tires are a scam" people. Sure, tons of people never buy winter tires and they're not dead. But if you've read everything I just explained there are obvious benefits and downsides and it's up to each person to weigh the balance of cost, safety, and convenience and make the decision that's best for them. Most of my customers I recommend owning two sets of tires to because that's what's best for their situation. Some of my customers do totally fine keeping their winter tires on all year, most wouldn't. Personally I have a Subaru that I don't use for road trips or regular highway commutes and I keep top of the line winters on it all year. My wife's truck runs all-terrain 3PMSF all year. We also usually have a cheap road trip car that's great on gas and keeps all-seasons on all year. The "all-seasons are just fine" people have usually never tried a good set of true winters though and haven't experienced just HOW MUCH of a difference they make. It's huge.