The better question is why does he wear a helmet ALL the time. It makes lots of sense when he goes into combat, but when he is just chillen on a starship he does not need it for anything.
The answer is still “intimidation” and “because he looks like a kid.”
Or to put it in simpler terms he wears it to feel powerful.
@impscum
Kylo froze Poe’s blaster bolt in place and put him into stasis. Later in the film he calmly deflected Rey’s shots away, put her into stasis and knocked her out via The Force. Lastly he completely decimated Finn in lightsaber combat. Just because he couldn’t defeat Rey in a weakened state doesn’t mean he can’t harm people at all.