Muscles are placed in groups. In males, they are more visible since they don't possess so much adipose tissue as females do, which, generally, renders a female body with rounder and foster shapes.
Knowing these structures will enable us to draw more convincing male characters. You don't need to learn the location or name of every human muscle, but you need to know how the main ones are placed and their volume. We need to know the muscles at their shoulders, biceps, chest and abdominal area, the so-called six-pack.