Are human armors even effective against demons, monster, giants, and other horrors in warhammer fantasy?

The empire states provide their troops with decent enough armor for their professional soldier as any army good in real life would do. The thing is I don't think an expensive plate armor could hold a single blow say from a troll or even a giant so why bother wearing them it just costs more money and weight? Orcs that are at least twice the size and strength of your normal citizen would definitely die from a hit with one of their swords which are bigger and stronger than a claymore.