A rounded, smooth, padded metal shell stops and deflects a whole lot thank you very much. And way more than your poor skull by its lonesome certainly. Not the really hardcore stuff - direct hits from heavy axes and maces for example - of course, but keeps your noggin intact against lighter weapons, glancing hits, arrows etc. by far better than nothing.

And that's just simple light "skull caps". Once the armourers get creative you find stuff like the later-end Roman helmets, Medieval "great bascinets" (a larger helm worn over a smaller one), visored helmets with optional extra armour plates and cleverly articulated "folding" face defenses... and that just in the European context.

Modern military gear has no bearing on the issue. Just as a little reminder tempered steel armour was still a valid defense against rifles in 1870.