Hiding your post history isn't only for bots and trolls
for a few reasons: for one thing, some redditors are absolute weirdos and go back through your post history and troll your previous comments and posts. another reason is that if, like me, you have an older account without a randomly generated username, you likely have a username you've used on other websites. I've been recognized on reddit on several occasions because I've used this username on other forums. I'm ok with being recognized, but I like to keep SOME semblance of anonymity between platforms... and a third reason I have my posts hidden is because I post in regional subreddits, and as already noted, I like at least feeling like I have a degree of anonymity, and I prefer for random redditors (or the trolls mentioned in my first point) to not know where I live unless they are also posters in those regional specific subs.
yes, unfortunately the feature leads to bots and karma farmers and trolls to hide their botness/etc... but I really like it for my own sense of anonymity/security. Ixm sure folks can still find my past posts, but at least it takes a bit more work and commitment to do that if they really want to.