The Imperial Guard are the Emperor's hammer of mortal men — the vast, uncounted soldiery that holds His galaxy one world at a time, not through any single warrior's might but through sheer disciplined mass. Every other faction fields fighters who are individually more than a man; the Guard field men, and win the only way men can. Their soldiers are the most expendable on any battlefield and the most numerous, a firing line of lasguns where rivals bring a handful of champions.
What makes that mass lethal is command. An officer does not win by fighting — he wins by making a hundred rifles fire as one, spending his own breath on Orders that turn scattered squads into a machine. Discipline holds the line together: the Commissar's pistol and the Priest's fury keep mortal men standing where terror should scatter them. Their durability is never a tough hide but numbers, cover, and the certainty that another rank waits behind the dead. Kill the officers and the machine falls apart — the whole strength of the Guard is a chain of command made of men who can be shot down like any other.