It doesn't, because you're clever and knowledgable about computers. Unfortunately, not all users are like you. Tell an ordinary user to search for an app on a search engine ("what's that? oh, you mean google!"), goto that apps website, download it, and run/install it, and before you know it he'll have a dozen Bonzi Buddies stealing his credit card numbers.
Most users need a web browser (email, FB, gossip sites), maybe a word processor, and maybe an occasional game. They need something more like Chrome OS, not an app store. Sophisticated users (gamers, programmers, tinkerers) don't need an app store either.
