Broadly speaking: 1. Aesthetics - you want to change some part of your physique by targeting certain muscles. Think doing curls for bigger biceps. 2. Performa

Broadly speaking: 1. Aesthetics - you want to change some part of your physique by targeting certain muscles. Think doing curls for bigger biceps. 2. Performance - you want to get stronger/improve mobility by targeting muscles or groups of muscles associated with a movement. Think doing hamstring curls to improve one's vertical jump or deadlift. 3. Therapeutic - you want to gain/regain/prevent the loss of capability. Something like deadlifts for back pain and posture or band pull aparts for shoulder pain. More specifically: Most strength and bodybuilding programs follow a template where each session has a main compound movement followed by accessory movements that target a handful of different muscles, usually split by upper/lower body or by type of movement (pushing/pulling) with the intent of working most of your muscles. Many non-compound movements for any given muscle are more or less interchangeable if you want variety in your program, have something that prevents you from doing a particular movement or just don't have access to the required equipment. For instance, overhead dumbbell tricep extensions give me elbow pain, so I do cable tricep extensions or skullcrushers if I don't have access to a cable machine.