Interpretation & Actual Result of "10% of your brain" Myth
It is correct that only a small percentage of neurons increase their activities  relative to their baseline level in response to   new stimuli or to more abstract thoughts. There are many computational advantages to this. For instance, in deep learning, sparse coding is a class of unsupervised methods for learning sets of over-complete bases to represent data efficiently. Basically you have more combinations to store patterns if you activate a few cells than if you either activate too few or too many.