The purpose of having the hopper guard in place is for safety. If you have the grill plugged in, the auger motor could start turning the auger at any time. If you have your hands in the hopper shifting around pellets or retrieving something that fell into the pellets, you run the risk of getting your fingers caught in the turning auger.
Replacement Hopper Guard/Screen for Traeger Pro 575/780 Pellet Grills.