From first principles, it seems surprising that a blow to the head would cause unconsciousness rather than just impaired function.
I've heard a lot of analogies to computers -- "Your brain has to reboot." But computers reboot when there is an error because they are explicitly designed to do so, in order to recover and prevent further damage.
So, did we evolve the capacity to get knocked out for similar reasons -- because it helped us recover or prevent further damage? Or is the capacity to get knocked out just an "accident" or an inevitable consequence of the way our brains are designed, without a good evolutionary explanation?
(I realize the answer might be: "we don't know". In that case, I'd wonder what the prevailing opinions are and what if anything we've ruled out).