This is an absolutely integral component of transformation, expansion, and growth.
We can understand mental metabolism as the process of sorting, processing, digesting, and eliminating the by products of thought.
Just like your body needs to eliminate waste products and toxins that are a natural by product of metabolism and living, so too should your mind eliminate toxins and by products of mental metabolism.
For example, if your goal is to lose weight, you have to eliminate the thought patterns of the over weight person and take on those of the ideal weight person.