It seems there are processes during brain development that 'do their thing' with can give rise (emergent property?) to an attempt to full (?) and complete (?) status?   It seems to be true that areas of the brain have their potential function lost by adjacent areas 'moving in' if it is unstimulated at the proper time or in the proper degree.

Yet other functions - language - develop no matter if the child is stimulated by human language or not.  

Removing functions in a 'finished' adult brain can kill or put a person in a coma.  But not always.  I don't know if an adult has ever had their corpus collosum removed.  There have been cases where it has been severed, eliminating the cross-hemisphere traffic, to ameliorate epileptic attacks.  These are the basis for the Split Hemisphere studies and their well-known findings.  These people are functional but not fully functional to a "normal" level.

As I said, it's a puzzlement.