You can ascertain Apple warranty status, week and year of manufacture with the serial number. It's understandable that a buyer might want to double-check that info.
It used to be possible to register a Mac to your account only using its serial number, but I don't believe that's true anymore, you now have to have the ability to log into iCloud on the device in question. So there isn't much potential for abuse currently, aside from maybe serial number spoofing (on Hackintoshes).