Yeah, I don't quite get how wearing one can stop something from going to someone else but can not stop from compromising yourself from someone else. Its the same cloth. How can it work outbound but not inbound?
Years ago when my wife was changing a diaper on our 6-month old baby boy, he inadvertently sprayed her face with urine. It happens!! If a clean diaper had already been in place, it would have reduced the baby's spray range, even though he might have still peed.
Now consider a dozen 6-month old boys playing together in a day care center. If no one is wearing a diaper, they might eventually pee on each other, unless you could keep them separated at a distance. But if they are all wearing a diaper, their pee will be somewhat contained, even when they get closer to each other. If their spraying urine droplets happened to contain a 'contagious' germ, wearing a diaper would be somewhat helpful to contain those germs from spreading so far.
Supposedly the coronavirus is spread by saliva droplets (like the baby's pee above) which inadvertently spew out from people whenever they cough, sneeze, or even talk. Wearing a cloth face mask works similar to a baby's diaper at reducing the range that those coronavirus droplets can travel toward other people.
Of course, the baby's diaper doesn't protect him if another baby's contaminated pee lands on him, but at least the diaper reduces the distance that any baby's urine can spray outbound.