Transferring purchases from iPhone to iTunes - Why is it necessary? When trying to restore my iPhone 5s to 7.0.4 via iTunes I get this message:


  There are purchased items on the iPhone “X’s iPhone” that have not been transferred to your iTunes library. You should transfer these items to your iTunes library before updating this iPhone. Are you sure you want to continue?


I know how to go about transferring the purchases by File → Devices → Transfer Purchases but what I don't understand is why it is necessary. I thought all my purchases were linked to my iTunes account. Can't I just re-download all my purchases from the iTunes App Store?

Is this to make sure apps that are no longer available in the app store are saved?
As you said, Apple knows what you have purchased and they are all linked to your account. However, transferring your purchases means that the purchased files (apps, music, movies…) will copy from your device (iPhone 5s) to your computer then you don't need to re-download them from the internet.