Most split routines work different muscle groups at different amounts during the week. For example, in the following split:
Triceps are worked pretty hard three times per week; first on Chest day during all the benching that occurs, next on shoulder day during the overhead pressing, and finally on arms day. Biceps are worked twice a week, heavy on arms day but they also get pretty fatigued after a hard back day. Shoulders are used both on Shoulder day as well as Back day during rows, and even on Chest day with incline bench presses. Meanwhile, legs and back are worked only once a week.
There seems to be a contradiction there; if working a muscle three times a week is optimal for growth for one muscle (in this example, triceps), then why wouldn't we just work every muscle three times per week? In a similar vein, why is a split routine (which, in almost every case, works the arms twice as much as the back, legs, and chest) preferable to a three-times-a-week full body routine that is more balanced?