Asking for a go-around from this position helps you in several ways. It allows you to examine any weaknesses in your student's piloting technique. For instance, you might see your student apply power and attempt to raise the flaps all at once, instead of doing so incrementally. Your student will certainly see the results of a poorly performed go-around when the airplane fails to accelerate properly (at least until you help him or her perform the proper technique). There's even a practical side to the maneuver. A Florida pilot once told me that he saw an alligator slither onto the runway as he began the landing flare. A properly performed go-around from only a few feet above the ground prevented damage to the airplane.