The masks won't stop everything, but they will stop some, and that could buy a person's immune system some time. The viruses will double their population in some amount of time. Going from one virus to a thousand takes 10 doublings. Going to a million takes another 10. And going to a billion takes another 10.
If you start with an infection of just one virus, it takes 30 doublings to get to a billion. If you start with a thousand, it only takes 20 doublings. So starting with fewer viruses gives your immune system a little longer to ramp up antibody production. If your immune system gets ahead of the virus (killing them faster than they replicate), you win. A little extra time for your immune system to get going might keep the number of viruses from overloading your system.
My conclusion today is: A bad mask is better than no mask.