If I have a great time with these games, I will probably double my payment next time. If I don't, I probably won't buy the next bundle at all. Either way, I can get my feet wet again with games with a low barrier-to-entry price.
Also, let's not forget that the cost of production of digital goods is essentially $0, and you could view this as an alternate channel for game developers to make some extra sales while getting people pumped about their future games.