The brain reacts to and absorbs the stimuli it is presented. When for a period of time, we turn-off the input, reduce the stimuli and go into a meditative state, we give the brain a rest. The brain is always on whether we are actively engaged or mindlessly watching a movie.
Quieting the mind through meditation is like consciously taking your thoughts on a luxury transportation ride in a stretch limo, relaxing by turning off brain activity but without losing touch with reality. It puts the meditator in a state of stillness where incitement from outside is met without attachment of thought. In the East, the birthplace of meditation, the end game is to reach a state of bliss referred to as Samadhi.