In a manner similar to how a breastplate raises the AC of your entire body. As a DM, if there was a rational reason for me to say something only happened at head-level (say a swinging log trap?) I might take into account who was or was not wearing a helmet.
In real life helmets are important because head trauma is exceedingly dangerous, causing both short term and long term injury. I think it is reasonable to generally penalize a character for not wearing one rather than scramble for specific instances. I mean, if you care about that sort of thing at all. "Realism" does not show up on the list of things 5e cares about.