In all the houses I've ever lived in, I've always locked the front & back doors, every night, without fail.
Why? If someone really wants in, all they have to do is throw a brick through the window and climb in. In some cases, even simpler - just break the glass beside the door, reach in, and turn the bolt.
Likewise, I lock my car every time I park in a public place.
Why? If someone really wants to steal my car or search it for valuables, he'll just break a window.
So why all the obsession with locking things up? It's not 100% foolproof, so why bother?
We bother because it "discourages the amateurs". Most thefts are crimes of opportunity - the thief can easily gain access, no one is looking, he can be in and out in a few seconds. Locking the doors prevents most thefts, though, again, not foolproof.
Why wear a mask? The virus can still get through, right?
Sure - masks are not 100% foolproof. No one has ever said they are perfectly effective at preventing transmission.
But they "discourage the amateurs" - in the case of the virus, they stop the majority of any exhaled virus from getting too far.
It's not perfect, it's not 100% effective - but it's the best, cheapest, most effective way to help (not fully prevent - help) control the spread. Even if X% of the population don't wear it correctly, the rest of the [100 - X]% still make things better.
It's literally the least you can do to try to control the spread of a deadly contagious disease.
By now I shouldn't be, but I continue to be amazed & depressed at how such an astonishingly high percentage of the population more highly values their own hurt sensibilities due to "it's wrong for the guv-mint to tell me what I gots to do" vs. any concern over spreading disease to vulnerable people.