How do deals with a devil actually *work*?
So I'm planning an adventure in which the players have expressed interest in making a deal with a devil. OK, cool, I'm on board with this, sounds like a good hook. 'Deal with the devil' is a classic storytelling trope and I've not really done much with it previously as a DM, so I'm game for this. So I went and had a look at the devil statblocks in the Monster Manual, and, um... I guess I don't really understand why anyone would do this?
The devils presented in the Monster Manual (bearded devil, erinyes, bone devil, etc) are generally presented as combat beasts and not much else. None of them seem to have any abilities that suggest that making any kind of deal or contract with them would really benefit anyone. They can't grant wishes like a Genie can, they can't provide beneficial spellcasting, so most devils don't really offer anything that a regular human mercenary couldn't. Unless the deal you want to strike with a devil is "Please go hit that guy with a sword", it's a little unclear what you'd potentially ask them for. And frankly, that seems like not a lot to ask for if your immortal soul is going to be on the line.
I wondered if maybe they're meant to be more tricksters, luring mortals into making bargains that don't really benefit them, but again nothing in the statblock really suggests this. None of the devils in the MM have shapeshifting, charm person, suggestion, hell most of them have decidedly mid Charisma and generally aren't proficient in Persuasion or Deception. Compared to the Lamia, which has a bunch of innate spellcasting to help deceive adventurers it interacts with, Devils have got basically nothing.
Most of them don't even have a way to travel to the material plane under their own power.
After all this reading I'm just really confused what a player would ask a devil for that they couldn't just get by asking any random sellsword, especially anything that justifies the incredible risk of putting your soul up as collateral. Am I missing something? Is the whole 'signing a fiendish contract' thing not actually associated with devils? Any advice is appreciated because I love the idea of doing this, but it kind of seems like I'm just going to have to homebrew a whole new bunch of devils or abilities for it to make any sense.