Whilst I agree that their effectiveness is limited, and they are more suited to catch outgoing particles from the wearer than to filter particles from someone else, I wouldn't dismiss them as totally useless. One thing they help to prevent is peoples' more subconscious practice of rubbing their noses and mouths with less-than-sanitary hands. Kind of like protecting yourself against yourself.

Additionally, it's far more common in Asia to wear a mask if you feel ill to help prevent spreading to others. So the use case here is 'I hope I'm not infected, but in case I am, I am trying to prevent you from getting infected'.