The Imperial Guard are the “Hammer of the Emperor” and the stalwart defence against the horrors of the universe. Each world raises regiments to serve in galactic wars and the units are named for their homeworld. Cadian 492nd (from the planet Cadia), Vitrian dragoons (vitria) Tanith First (from Tanith) and so on. These are the typical men and women - mere humans - who fight to preserve the Empire. No superhuman space marines or gifted Inquisitors here just men like you and me.
as @Slayerkin described, chaos is more insidious in W40k, having cultists and corruption at planetary level. If the Planetary Defence Force, the Planetary Governor or the Imperial Guard stationed on that planet turn to chaos, they keep their equipment and fight against their former masters. This is what appears to be on the cover with an imperial guardsman, clearly corrupted with the chaos sigil on his helmet. It gives hope for lots of ranged firefights (for me at least).