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Nk9bjP4A "Why do ninjas have to cover their mouth and noses before entering battle?"
1. To hide their identity
2. To protect themselves from the effects of their own smoke bombs
3. Possibly to intimidate as well(as they used to spread rumors that Ninja were demons capable of using magic and all sort or bad things)
These are the biggest possibilities I can think of. Of which the 1st one is obvious since some of them would also work while in disguise, having their real faces known to the public. Some of which would disguise as guards, gardeners, servants, traveling doctors, tea shop employees and sometimes even prostitutes.