Is there any real reason for this? I can see how it might allow more time for recovery if you are only focusing on one body part per day, but anytime you are doing compound movements, those muscles will be used anyways amirite? I've never read/seen any discussion on this, so I'm wondering why most people have dedicated days. my rationale: I try to do all the major muscle groups for one to two exercise each time i visit the jim. That way, I can adjust accordingly on what exercises to do depending how I feel. If I am exhausted or low energy I generally will save squatting or benching for another day. If I am feeling full of energy and well rested I try to knock out as many compound movement exercises as possible with challenging weights. It also prevents me from totally omitting a muscle group if I miss a day in the gym that I can't make up.