It could be that you are infected with Covid-19 but are not showing any symptoms. You don’t know you’re infected, so you could pass the new coronavirus on to others unnoticed.
In order to protect others from infection, you have to wear a face mask. The recommendation is for as many people as possible to wear a face mask, because this at least enables droplets and particles of mucus to be caught to some extent. The chain of infection is broken.