In my experience the "bounty" is faster attention to the issue from the devs (no-one likes to spend a lot of extra effort on trying to work on something that is vague, ambiguous or confusing), a better chance that a fix is created quickly and addresses your actual issue, and that it creates a good working relationships with the devs for working with them in the future (builds karma). I like to think of it like keeping up my end of the implicit user/Dev contract :)