We sit to regulate our system from the nearly constant mental/emotional/judging activity that directly activates our physiological fight/flight response.
We sit to practice being with all our internal machinations without reacting to or acting on them so much.
We sit to find equilibrium, to reset, and to act more skillfully and kindly.
We sit to practice being alive and aware, here and now.
We sit, not to attain our Buddha Nature, but to express it.
We sit because we love to sit, and we do it for the whole world. (And the whole world is sitting us!)
Why does that frog sit on the lily pad, not waiting?