It's my understanding that the purpose is to protect data. It has a secondary effect of preventing use of the device. The goal of reuse and preventing device use are incompatible.
But most of us don't want to extend the courtesy to petty criminals.
Given Apple's staggering pile of cash, I think that we'll have decades of warning if they're going to stop existing.
Even though at any point said device can be unlocked, transferred, sold etc.
It is to stop devices being stolen, wiped and re-sold. It is an attempt to allow the legal owner of the device the ability to control who can use it, rather than the possessor of the device having full control who can use it.
Its only purpose is to protect the use of the device by people removing all the software (erasing back to factory defaults) and taking control of the physical device that way.