If a human brain is not flexible, it will be a lot more limited than it already is. In another way of looking at it, you can even say that if it does not expand, it is only a box or a storage. Our brains do not only work as storage of information! We also use our brains for thinking etc. If it is rigid, when a lad hit you accidentally on the face with a paper, you may resort to violent retaliation.
Brains should be adaptable on the grounds that on the off chance that they weren't we could never have the capacity to expand our astuteness. Individuals are more adaptable mentally on the grounds that we can turn our hands to more mind boggling things and can learn better.