If a slow-growing left-hemisphere tumor disrupts language, the right hemisphere may take over this language functioning. this best illustrates the value of

I believe the answer is: plasticity

Other example would be the increased hearing among blind people. Let's say for average human x% of our brain would be allocated to maintain our eye sight.

When we become blind, this percentage of our brain become free since it no longer had any workload. Because of this, the percentage of the brain could be used by the part of brain who operates our hearing mechanism and enhance its function.