They are only useless when half the people don't use them and ignore all other measures while they're at it. Obviously there's no need to wear it at home; that's just stupid. But outside, in public space, it catches a lot of those particles that can carry the virus. If everybody wears one, that means less virus particles in the air, and less chance of infection.

And during the pandemic, a lot of countries did indeed not have their usual flu season.