It is just an idea. There could be once a month voting for "best" member of the month. It would be just a way to say thanks for all the help. Every member could for example give 3 votes for others (not possible to self vote).
Criteria for giving a vote is up to personal preference. Someone will vote for the member that was most helpful to him, someone for who is writing good quality answers, etc... Over large number of voters this personal preferences give good average.