The games change every so often, you pay only what you want to pay even if that's just $0.01 if you pay over the average (usually less then $5) you get some bonus games. Your donation also goes to a really amazing charity called Child's Play and Electronic Frontier Foundation You also get to choose how much of your donation is split among the games your buying and what goes to child's play and the eff.
Often times a game will be a pre beta or alpha debut. Meaning you get to play and support the game as it's in development. Other games that have done this are minecraft, and lords of uberdark. So it's a very nice way to get cheap games, help out indie developers and support some very worthy organizations.