Many students do report improvements in mental clarity. Every muscle in the body has nerve connections leading to and from the brain. When tension is subconsciously held in many muscles throughout the body, the mental "background" noise is increased. By eliminating this nervous energy, a deeper clarity of thought is achieved.
Tension uses up the capacity of the neural interconnect system. When a lot of effort is put into tension patterns, creative thought is thwarted. If clear thinking is likened to "clear skies", then tension is the "smog" that reduces our internal visibility.