Never gave that a thought. Erosion control. And then, if and when you do till it up the decaying grass enriches the soil. K0, you taught me something today. Thank You!
Of course, there's nothing wrong with not tilling it up and leaving it as a grass start.
In my last motocross race there, I don't remember it being grassy at all, and in fact was quite dusty. But what I remembered the most was on my first moto I came off the gate hard expecting wheel spin and roost, and instead my tire hooked up and I wheelied almost out of control. Had to let off to recover. Ruined that start. (not that it makes a difference in my race results anyway, but I like to lie to myself that it does
).