In World War I & II footage it is standard to always see soldiers with helmets as part of their attire. Was there any physical benefit to wearing a helmet (i.e. did wearing a helmet often actually save lives in the hell that was trench warfare and artillery fire?), or was the helmet more of a psychological benefit to the soldier, to allow the soldier to not feel naked among the fire?
They were effective at preventing head injuries and could stop subsonic pistol rounds. If you look at WWII tankers, British tankers suffered more head injuries than the Americans because the British tankers wore berets whereas the Americans wore helmets.
Helmets could not stop a rifle round or even 9mm. They could stop some shrapnel.