A lot of members of society view not wearing one as an act of courage. A friend's mother will tell anyone she encounters who is wearing one that they are a ninny and a coward. So for some people it is also an indicator of courage/weakness.
A study lets me tell her she is an idiot. Preventing others from being called also cuts into the "ninny" narrative.
It seems like skin contact isn't quite so likely to spread the virus but breathing in exhaled droplets is.
Incidental interaction doesn't seem to spread the virus that well, so wearing a mask significantly reduces the risk to people like your grocery clerk.
It seems like general contagiousness isn't the primary mode of transmission, but instead the vast majority of cases come from superspreaders.
All of those indicate that mask wearing is probably more beneficial than expected.
If skin contact were the primary mode, then masks would likely have downsides as they tend to provide a warm, wet spot for the virus to hang out until you touch it.