A good troll will make it an entertaining place to be. The benefit to themselves is probably just fun. Unfortunately a lot of the trolls are not good trolls.

they scare off wandering adventurers and are good for protecting treasure in your dungeons.

I guess the negative attention makes them feel good as they are unable to get any positive attention.

You're able to get reactions out of others

The health insurance is great.

it's funny

I guess being a troll allows one to pretend others are inadequate so one doesn't have to face the fact that they themselves are.