You could say "He needs it so Headcrabs don't latch on! Or He needs to breath in Xen! Or he needs it for the radiation! Or the HUD!" Lets start with the Headcrab thing. We've only seen a Headcrab latch on to someones head TWICE, one time in Half-Life and another time in Half-life 2, headcrabs never latch on again after that. Now with the Xen thing, well the Xen aliens have no problem breathing on earth so why would Gordon have troubles breathing in Xen? Now with the radiation, Gordon swims in it and you gotta remember that for Gordon, these games take place over a small period of time. The first game takes place in about 2 days, the second is played with 3 with the teleporter time jump in the middle and the episodes take a day at most so i think Gordon can survive that entire time without contracting brain cancer. The most radioactive places in the games are the Anti-Mass Spectromater and the core of the citadel but the Anti-Mass Spectromator doesn't set off the suits geiger and the core is just a big ball of combine dark energy so who knows how that works, the stalkers are running around naked and they're just fine. Now with the HUD, in HL 1 - 2 the HUD only appears when Gordon get the suit implying its the source and if we're assuming that the most logical place from it to come from is inside the helmet, that would explain why the player always see's it at the corner of their eye, Lets pretend for a moment that the gearbox games don't count because in both BS (Blue Shift) and Decay because they dont have a HEV helmet, so the explanation of the HUD, it's not actually there, its just for the player to tell how much health and ammo they have like in every other shooter but beyond that the HUD can be a visual representation of the suits voicelines. the HL series unrealistic aspects for the sake of gameplay. Like how his glasses are always clean, how he carries all these weapons around, and infinite bug balls. But I know what your thinking "These are points why he doesnt need the helmet, prove that he actually doesnt have one" well in promotional art, console commands in HL1, Blue shift and Opposing Force, tie-ins with other games. Not once is he seen wearing a helmet but if only there was one game where you didn't have to configure the game to see Gordon (spoilers for hl:a ahead) in the ending Alyx gets a glimpse of gordon in the HEV suit he's shrouded in shadow but you can make out his hair and glasses, with no helmet and even in testing of the ending of HL:A they use the same model for him with no helmet. Case CLOSED! (Credits to purplecolonel for inspiration and some of the lines in this rant, heres the official vid Does Gordon Freeman Wear a Helmet? | Cascade by Purple Colonel)