To practice drawing the structure of your facial features; and to practice the placement of your features.
Doing a composite face drawing really helped me learn and see my misconceptions of how the face looks in my head like how i make the eyes to far apart or the nose bridge too long or the head to big for the facial features, the list go's on and on and thats why its so important if i had not done this I wouldn't have seen how unrealistic and disproportionate my idea of a face was this helped grounded me more in the reality of what a face was supposed to be and not just how i thought about it in my head.