I've read such conflicting information on them. Some sources say they protect other people from you more than they do you, some say they protect you 60-80%, some say washing your hands properly (and frequently) is more effective etc.
You need to wash your hands to supplement a mask. If you believe it's working, the mask is now covered in infectious fomites, and you need to get it off you without infecting yourself.
The good news is coronaviruses are enveloped, and thus relatively susceptible for disinfectants.