Shawn Blanc, in what has to be the best article I’ve read about the Mac App Store:
The problem had nothing to do with availability or quality of 3rd-party Mac software. To the contrary, OS X has an outstanding community of 3rd-party developers. You and I have no problem finding and using new tools to make our day-to-day computing experience better, but the average consumer does.
And so Apple wants to introduce the non-nerdy to all the fantastic software that is available for OS X. Which is precisely the goal of Mac App Store.
Seriously, go read the whole thing.