@Amar__
It's kind of like if the whole country had AIDS or HIV -
It would make sense then to make everyone wear a condom to stop that sexual spread by protecting the sexual way.
Likewise Covid is spread from the mouth and lungs so that is protected to stop sending the infection just as much as it could protect you a bit by wearing a mask too.
Plus, the mask is not the only thing that happens - when a whole country is following wearing a mask (like the country I'm currently visiting) - they do things like check your temperature at every single shop you visit, meaning that if you are showing signs of illness then you are not coming in.