It's well known that to actually defeat evil, you have to directly kill the Big Bad -- you can't just hack through enough of his soldiers to leave him relatively harmless. So the army of good will stall the army of evil while the hero and his friends sneak in to kill the Big Bad. This trope can be justified if the Mooks were not loyal to the villain in the first place; they may have been forced to fight against their will, or obeyed out of fear of death or a Fate Worse Than Death. Alternatively, without the unifying figure of the Big Bad, The Empire would rapidly collapse into warlordism.