I'm sure there are reasons, but it stings a little.
I've never actually traded in any of my old phones, on the fear that the new one might break at some point and I'd need a backup. Granted I've only once had to take advantage of this in the 13 years I've had smartphones.
I didn't click all the way through, but I'm hoping you don't have to turn in the old phone immediately... I'd like to hang onto it for at least a couple weeks just in case I find the backup/restore process hasn't completely done its job.