Yes face masks are pretty effective at preventing the spread of COVID-19.
If someone is infected with COVID-19 and then coughs or sneezes without the face mask and you're around them but are wearing your face mask then it is less likely that you'd get infected as the virus could not easily get through the face mask.
Also if the infected person who has COVID-19 has the face mask covering their mouth and nose and sneezes or coughs then the virus will stay in the mask and not get out into the air.
The virus would then die soon in the face mask.
Always wear a face mask to prevent the spread of COVID-19.
When you're outside and alone there's no need or reason to wear a mask.
If you do have a virus such as Covid-19 or the flu or cold virus and sneeze or cough and nobody is around the virus will just leave your body and go through the air.
The virus will not survive long and eventually the virus will no longer be in the air to infect anyone else.
Although it's still a good idea to wash your hands often and use some hand sanitizer especially if you do sneeze or cough.
I would recommend having a face mask in your pocket or purse and have it available should you need it when someone comes up to you or you need to go into a store.
I always keep a face mask in my purse just in case I need it.
But when outside or in your own vehicle you do not need a face mask.