The point is not that it gives you perfect protection or even n95 protection. It does give you more protection than maskless. Also the lower the viral load you're exposed to, the longer your body has to prepare. How the disease progresses is dosage dependant.

A low enough load is like a natural vaccination. You don't get sick, but you get some immunity. A high enough load and you can get the sickness very severely. It's a gradient, and masks help put you lower on that gradient than otherwise.