I can come up with three reasons. First is to remove beef residue before cooking chicken. Second is to prevent chicken from sticking - a clean grill with fresh

I can come up with three reasons. First is to remove beef residue before cooking chicken. Second is to prevent chicken from sticking - a clean grill with fresh oil sprayed on it works best. Third is that, when running a grill for a number of hours at a time, you generally can't afford to shut the grill off to clean it.