Not defending the practice in a combat scenario because that's obviously dumb especially in the uber warfare that's happening in 40k. Virus bombs that can destroy entire ecosystems in proverbial seconds, rounds and shells with more diameter than a fucking bus, chaos sorcery of the worst kind is a frequent encounter as well.

But for propaganda material it makes a lot of sense I'd say, the masses need to see their heroes' faces. There are also some marine chapters that use mutated or otherwise extreme gene seed when making their warriors, for example one chapter becomes a lot more animalistic in its physical characteristics and the helmet negatively impacts their altered natural senses.