A kick reaches twice as far as a punch and carries the weight of the leg and hip behind it, which makes it the strongest opening an unarmed fighter has against someone who has not closed yet.
What it cannot do is reach high. A kick aims low, at the legs and the body, and a head is simply out of its way — the low aim is a property of the attack, not a choice. It is also slow to recover from: a leg in the air is a leg not holding you up, which is why a kick that misses so often ends with the kicker on the ground.
A rigid helm, boot or gauntlet adds 2 to the impact — which is why a gauntleted fist is worth so much more than a bare one. Thrown with an off-hand or off-foot, the strike takes −10 and loses a point of impact.