I know someone who wears one because a lifetime of depression caused them not to take care of their teeth, and so they don't want to be judged even more poorly for that.
In the UK we have the Criminal Justice and Public Order Act 1994 section 60aa where the police are able to make anyone within an area remove a mask when a there is a large crowd and a senior cop decides to fill in the paperaorj. There has been a recent increase in its usage in the last 4-ish years.