Useful to anyone even remotely involved in the process. The random helping hand gets that warm fuzzy feeling of helping, and nerd credz. I've done this before, it really is a motivator. The sole maintainer gets assisted with their roadblock. All users/watchers of the project get to rejoice because the project has progressed.

I do not see why you focus so much on team building when that is completely unrelated to what is being discussed. Not all open source projects are big enough to warrant any kind of team. They're literally just someone's side project, which may or may not be useful to other people.