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human
[Warhammer 40k] Why do some Space Marines choose not to wear helmets in battle?

In many depictions of Space Marine battles, it's not uncommon to see officers and others without helmets. While some few have undoubtedly lost their helmets in battle, others have not. Forgoing a helmet seems to be a choice. Why? Doesn't the lack of a helmet lower a marine's survivability and thus their ability to do their duty to the Emperor?
gpt
There are several reasons why a Space Marine might go helmetless into battle. Some Chapters allow it because it projects an aura of fearlessness and invulnerability, both inspiring nearby troops (usually the guard) and demoralizing enemies. Some marines believe that their augmented natural senses will give them an edge in combat that would otherwise be nullified by the helmet. 

More often however, modern artists in the Empire prefer to depict marines without their helmets because it allows an average viewer to better identify with and understand the nature of the glorious acts being portrayed. You can bet that when Marines purge a Necron ship they are all wearing helmets (Necrons have little need for atmosphere in space) but the paintings commemorating their heroism will show them helmetless for effect.