If you want to make a patch, you need a starting file and an end point. So you take a pristine tree and call it "foo.orig" or "foo.old" and then run diff -ur foo.old foo

Maybe younger developers don't know this experience because usually their patches are generated by git diff and the like. But before that stuff was as common as today, you would need to copy the old files off somewhere to generate a patch. At that point you need to name it something.