[mathias_10gen]

You actually list one of the advantages of GridFS right there in your post: streaming. If you are serving up a 700MB video, you don't want to have to load the whole thing into memory or push the whole thing to the app server before you can start streaming. Since we break the files into chunks, you can start sending data as soon as the first chunk (256k by default) is loaded, and only need to have a little bit in ram at any given moment. (Although obviously the more you have in ram, the faster you will be able to serve files)