Several reasons for it: helps you maintain a fighter's rhythm (common in a lot of striking arts, actually, although often expressed differently), encourages a step-drag-step-drag gait to maintain base of support, keeps you on the balls of your feet to facilitate mobility (while discouraging flat-footing, which makes you a sitting duck), keeps you light on your lead leg for the teep and checking kicks, facilitates weight shift if you're going to throw the rear leg roundhouse. You don't do it because it's cool, you do it because it feels natural and is functional. Different fighters keep different rhythms, typically, and the the rear leg can tap too, often more subtly.