Much like most of the mythos surrounding Michael Myers, the reason behind his mask is not entirely clear. Michael is designed to be a mystery and even Halloween director John Carpenter has left much of the whys behind Michael's character unanswered. So, the reasoning behind Michael's mask can be looked at in two ways.
The first is the natural discomfort people feel when they encounter someone in a mask. People often associate masks with fear, and it's because it plays on an inherent fear of the unknown. A mask is designed to mimic human (usually) visages and expressions, but they are emotionless and hide the truth of what is beneath them. Even if the mask is pleasant or beautiful, it can still be unsettling. For that reason, masked killers in horror films became a staple. Concealing the identity of a person makes them even more terrifying, right?
The second reason Michael may wear a mask is a bit more personal. Michael was designed to be The Shape. He is, for narrative purposes, not human. As far as we know, Michael does not even see himself as human. Described as the "embodiment of evil" by Laurie and many other residents of Haddonfield, Michael wearing a mask to cover up the person he used to be makes a lot of sense. He loses any sense of the identity he was given at birth and, instead, his mask becomes his identity. This may be why he has such a violent reaction to losing the mask. For example, in Halloween (2018), two true crime podcasters visit him at the sanitarium before he escapes. They manage to take his mask and taunt him with it in hopes it will get him to talk. He later tracks them down, brutally murders them, and reclaims his mask before doing anything else.