Why do I have to "transfer purchases" from my iPad when syncing to iTunes? I just started syncing my iPad to my desktop iTunes and it's taking a very long time to "transfer purchases" from the iPad. I was under the impression that iTunes remembered all my app downloads and purchases via iCloud (and verified I could see them in iTunes) so I don't understand why iTunes would need to transfer anything off my physical device.

What exactly is it doing and why is it necessary?
This process assumes that iCloud is not enabled.

It is possible to update apps on the device without doing so in iTunes. For this to work you will need to have enabled automatic downloads.

If you sync your iPad with iTunes without transfering purchases you coukld end up with older apps and losing the updates.

With iCloud there is no need for you to sync with iTunes, so if a device is plugged in iTunes would assume that you do not use iCloud and will transfer all the updated apps to the computer.

All in all it is more of a safety precaution if you are already using iCloud.