Are you talking about OE body parts?
Not all manufacturers do it, but GM does a lot for some reason. My guess is to keep it away from LKQ/Keystone, but I've never had LKQ/Keystone ask me for smashed up junk? We don't even have to provide GM with the part, just keep whatever pieces of it we have, and scrap it when they tell us to. We have a bunch of pieces of bumper covers in the back with part labels on them.
I hate it, more silly hoops to jump through and just another reason I turn most GM collision customers away/refer to a larger dealer. Makes it not worth my time.
One good thing about Hyundai/Kia, they only do non-warranty cores on stuff they will actually rebuild.