Or is the small part the bit where the least useful stuff happens? ;-)

A bit more seriously, the brain actually uses a significant chunk of the body's energy budget (no matter how efficient it is relative to human made equipment). So evolutionarily, it doesn't make sense unless it's doing something exceptionally useful.

No, definitely not. Our consciousness is honestly an afterthought, as the brain processes mountains of information that does not even get to out conscious level yet is arguably more useful than anything we primitive primates can think of.

It's the part that makes everything useful happen but most of everything useful happens in unconscious parts of the brain and even outside of the brain.