By now I think most gamers have bought something from an App-store. Steam, Xbox live Marketplace, iTunes or whatever.
Furthermore I think a lot of people have bought games from places like Apple's app-store but do not consider themselves gamers. So app stores are all the rage now and with Steam now available on the mac it becomes the de-facto PC place to buy games. proprietary platforms all will come with their own. It's seen a boom in independent and low budget games going through a huge surge in market that hasn't been seen since the shareware days.
But even more so for games than other software. Look at the statistics from apple's apps and you find games are number 2 in size. Second only to books which may get kicked out now apple has their own iReader system. So all these app stores regardless of platform or format are a boon for games.
But why games? Honestly I'd love to know myself. There's something about open source that has made it difficult for games to get traction but hasn't been a problem for lots of other software. So this paid model allows lots of wanna-be-game-developers to give it a try with minimal investment and risks. Combine that with the affordable luxury of buying a game for a few dollars instead of over a hundred and you have a thriving marketplace. And good on them! I look forward to seeing where this goes.