One point, following Steve's reply, is the need to allow recovery between some workouts. If you look at those that train as many as five days a week you'll see that the workouts are, for the most part, structured in such a way as to allow some recovery. Now no workout is completely isolated from the others but, by way of example:
1) (say Monday) chest and back
2) (say Wednesday) Legs and calves
3) (say Friday) Shoulders and arms.
You've a day off between workouts and the shoulder joint gets Tues, Wed and Thurs to recover. Then it also gets, as does the whole body, the weekend too. Legs, which are brutal to train properly, get a whole week.
Other versions will try to work the same kind of breaks in even if you train 5 days a week.