1.) To somewhat retain the appearance of a Lawn Mower.
2.) As they are configured on the prepared class, they serve both as nerf bars, preventing other mowers wheels from running in between yours, and as additional area for foot rest.
Under current USLMRA classification this would fit everything but FX, and there you are allowed to fabricate the nerf bars/footrest to suggest a deck or run a deck like the prepared classes.
They don't really get in the way because most run deck halves welded to the factory running boards. They're not full decks, and they don't extend beyond the wheels.
