The Single Leg Stance starts with standing and feeling balanced. Once that is achieved, the Punching is used to build a root. This Root is necessary for the development of kicking power. After kicking is developed, these skills are used to improve the stepping. The Single Leg Stance is also the first Stance which develops the sesitivity to the energetic qualities of the root. Obviously you can focus at any time on any thing, but the Single Leg requires the focus. Without Focus, the Stance falls apart in the slightest breeze.
Many styles use the Single Leg Stance for their hardest hitting punches. Mainly because You might fall over if it is unsucceessful. A Bow Stance would usually break the hand where the Single Leg would cause you to fall back or to the side without injury to yourself. A useful quality to have during a fight. Obviously a broken hand is not going to strike again. As the styles improve, the leaping single leg is phased out and more stable stances are used. This requires that the arms and hands become stronger which usually comes from iron palm training and body hardening.