To hold the baby down in the seat. To hold the harness straps on the shoulders. In vehicles with contoured bench seats. In the rear center if the seat has a hump. All of the above.
Boosters are comfortable for children because they allow their legs to bend normally.
Most 40 pound children are not tall enough for combination lap and shoulder belts to fit properly. Many young children will not sit still enough to keep lap belts low on their hips. All of the above.