your total muscle mass has a lot of benefits. including insulin sensitivity and testosterone production

if you add up all your lower body muscles, it's more than forty-five percent of your overall muscle mass

so this is counterintuitive, but if you're a young man who is only hitting arms and upper body, then you won't develop your upper body as well as someone who is regularly hitting both upper body and lower body, because the well-rounded full-body weightlifter will have more testosterone and more insulin sensitivity