I used to work in health care, and while I'm no expert, I can offer up this analogy as to why mask-wearing is effective.
Imagine that you are in a nudist colony and someone starts peeing on your leg - That is the equivalent of both being unmasked.
Now you put on your jeans, you will still get wet, but not nearly as bad - That is the equivalent of you wearing a mask
Now, they put on their jeans - that is the equivalent of both persons wearing a mask.
The virus can pass through yes. But it is transmitted. It's not like pollen or spores that float out in space on it's own. It's resides in body fluid like saliva or nasal fluid. Those droplets are caught by masks.
Hope this helps.
