OverGrow has a 'karma' system. You get 'points', from other users negative or positive based on your posts. Like if you made an informative or helpful post others can give you positive points for it, if you made an offensive childish post you would get negative points.
The only other purpose I see in this (other than it being a popularity contest) is to give frequent posters credibility.
Esp. in the grow section a member who obviously has a lot of "points" his posts could be taken seriously and would be a major help to less experienced growers