What is the purpose of being strong?  Emergencies aside, to be the best human you can be.  A quick mind requires a quick body.  To be healthy in cognition requires both practice and a healthy body.    In other words, intelligence, the one thing we have over the other animals, requires a healthy working brain.  That brain needs proper nutrition and oxygen delivery.  You don't get that with a body that's so fat it can't get out of its own way.