brain capacity is advantagous for bi-pedal life style and humans in general because it is 3x larger than the chimp's at 1200cc (a chimp's is 400cc). Because it is so big, the skull is large which creates a larger surface area for the attachment of special muscles that control the shoulders and spine. Our brains delevop at embryonic rate for one year after birth, where as chimp's brains develop very fast in gestation and slow down at birth. This means we learn alont in our first year of life because our brain develops at the same rate as if we were in the womb but there is no sensory protection. We can't hold onto our mothers the way chimps do at birth even with hand reflexes. Mainly because our mothers have no fur because the sweat because of our big brain.
we are the only species with a bi-pedal lifestyle so really when we talk about the brain we have to say why it is selectivly advantagous to our species' survival.
http://www.whyevolution.com/chimps.html
heaps of info here - you really need to skim through it... right down the end there is heaps of good info about everything we need there are also some good comparitative pictures