To lift a weight is to move it away from the ground. Functional movements usually do this from a standing position with the feet on the ground. Having one’s weight on the heels results in better force transfer to the weight than if the weight were forward on the toes.

Violation of this principle is often an indicator that you’re doing something wrong. In the air squat (and its weighted variations), is an indication that you’re not pushing your butt backwards enough at the beginning.