The reason for the start page is to make the desktop look scary. Average users don't want to drop to the command line for any reason, the black box with cryptic commands looks scary and isn't intuitive to them. MS is introducing another layer on top of the desktop that really simplifies things. When an average user launches a traditional app, they will be dropped to the desktop, which will seem scary to them, the apps that run in that "mode" aren't as simple as they're used to, there's menus on top and tons of toolbar buttons instead of a dumbed down phone interface. This will force developers to adapt, because users will no longer want to run traditional desktop apps anymore, it'll seem too complicated.
Previously, users were forced to learn this stuff, but now that they know there's a simpler alternative, they won't want to, just like the command line. This benefits MS in that there will be a ton of new apps that work perfectly on their tablet. This gives an incentive to app developers; They will now have a reason to sell you the latest version. It benefits the Windows platform in general because the new users that are attracted won't be able to cope with a traditional desktop interface, and other OSes will look scary. More experienced users will know how to get around this stuff and run traditional apps, and won't be bothered too much.
Yeah, it seems really stupid to most of us, but we won't use it, but there are many business reasons for MS to force this start page and tablet interface onto users, it feeds into their new tablet strategy and throws developers a bone, and gives them a reason to focus on MS's tablet platform the way they do on iOS even without a large pre-existing userbase, simply because now average desktop users will be demanding apps in this format. So why shouldn't they do this?
