So you can concentrate your energy on maintenance tasks, like memory consolidation and physical restoration, during the down time during which it is cold and dark -- when you can't effectively hunt and stay warm. If you did not do that, you would have to constantly allocate some energy towards these tasks, at the expense of more urgent ones. And the higher up you are in the food chain, the less risky it is to rest.

Which maintenance tasks, why does it require us to be unconscious, why can't we apply constant energy towards them? Lots of interesting questions to answer