I suppose besides a "good, clean detonation," the plate may also serve to keep whatever it is on top of (concrete, perhaps?) from turning into potentially lethal projectiles.
Well, I did notice that many were wearing respirators of some sort and heavy gloves. I'll bet there's even some steel-toed boots being worn as well.
ETA: I'm not sure that they're on concrete, so the plate might just serve as a solid and flat place to strike, improving chances for complete detonation.