"Transferring purchases" Whats the point?

When I sync my iPhone to my macbook, it takes quite a while, and it appears that most of the time is spent "transfering purchases." It spends all this time transferring apps to my mac. Is this just my app data or is it the entire application? If its just the app data, that should be a really quick process I would imagine. Also, app data is backed up as part of the "backup" process, either through wifi or via the separate backup this phone option. If its the entire app, then what is the point? I can't execute ios apps on my laptop, and if my phone gets erased, why couldn't it just automatically re-download all of the apps I previously had installed when I put my Apple ID back in and the phone can see what apps I previously had installed on my device? Seems like a waste of time to me.

I can understand letting itunes know which songs/books/videos/tv shows, etc. that I purchased on my phone, so that I can download them on my computer also, but that still wouldn't require downloading the actual data, and it wouldn't have to wait until a sync session to do that. Both the phone and itunes on the computer are listening to Apple's servers based on your Apple ID for information on what media you've purchased.

So bottom line is what is the point of downloading ios apps to the computer?