Why is the brain's cerebrum split into two hemispheres?
It is simply the division of labour , we can say so as to explain why cerebrum is divided into two cerebral hemispheres.
Cerebrum is divided into two cerebral hemispheres by longitudinal fissure and are connected by Corpus callosum.
Cerebral cortex has many folds which increases surface area of brain is an indicative of higher evolution in us.
Functionally , left cerebral hemisphere controls right side of body and right cerebral hemisphere controls left side of body.