I think mostly so that they can act like bookmarks, only better. I have a Google Docs icon on my desktop that will launch Chrome and go straight to my documents. Sure, I could drop a bookmark there just as easily, but by default it'll have the logo of my default web browser. Sure, I can change it, but it's a hassle. Also, since it's a "Chrome app", it can do a lot of things that apps can that URL links can't. It's easy to pin it to the start menu and/or the task bar.
Then there's also the Chrome App Launcher, which is a convenient menu of all of your Google apps.
It seems to me that they're really just for convenience. They're certainly not essential. If you can live without them, great.
(I think there was some speculation that having these apps might make working offline work or work better, but it doesn't seem like that's what's going on.)