Armor Was Designed for Protection Against Glancing Blows
[Steel] armor was more protective than bronze. A steel sword striking a steel helmet was more likely to skip off or fail to bite, so more effort would be made to hit the enemy in the unprotected area, shoulders for instance, than on the head.
However, with bronze it’s different. Bronze helmets are not as thick and protective. A hard blow with a bronze sword could crack or crush the helmet. The sword would be only slightly damaged, especially if it was one with a thicker edge. Armor and helmets were designed for protection against glancing blows, and not for well aimed full force hits. I imagine in the heat of battle there would be a lot of glancing blows. Blows would be coming from all directions, even from those on your own side. Swords would be knocked aside, bounce off of shields, rebound right and left, and be thrown up in spasms as someone was hit and killed. We know that such combat took place from the Iliad and the Odyssey, not to mention pictorial representation on vases, and from other written sources. In short, armor was needed not only as protection from your enemies, but your friends as well. It could not give you complete protection, but it was a lot better to have some protection than none at all.
— The Book of Swords, pp. 20-21