You help muscle growth all over your body by using heavy compound lifts. The two exercises that provide the biggest bang for your buck are full depth squats (to parallel) and deadlifts. These exercises target:
- Upper and lower back
- adductors and abductors (squats)
- forearms (grip on the deadlifts)
- biceps (supporting bar on squats)
So you are getting work across your entire body. If you add to that a pair of compound pressing movements: the bench press (everyone's favorite), and the standing overhead press, you will have hit just about every muscle group in your body.
The big reason why:
Squats and deadlifts provide the highest testosterone response. Testosterone in turn triggers growth hormone and insulin like growth factor 1 which are major muscle builders for the body. Every muscle that needs to improve will get the hormonal response to trigger growth. Assuming you have the protein reserves necessary to build up the muscle, that's what will happen.
Combine heavy training with proper rest, and you have a recipe for getting bigger and stronger.
How often to train:
If you are a beginner--meaning you can recover completely every session--there's no reason not to squat every time you get in the gym. In fact two of the best beginner lifting programs have you squat every session, and alternate between bench press and overhead press. The only difference between the two as far as exercise selection goes is one alternates between deadlift and power cleans (Starting Strength), and the other alternates between deadlift and barbell rows (Strong Lifts).
More advanced trainees might be better served to limit squatting and deadlifts to once per week. The reason for that is because it takes longer to fully recover from the effort. Wendler 5-3-1 program has you split your main compound lifts (squat, bench, deadlift, overhead press) so that you are focusing on one per session. The assistance work you do in each session is selected based on your desired goals--such as getting strong, building mass, etc.