Assuming a mask blocks 50% of particles or droplets in either direction (preventing yours from escaping if you’re sick, preventing outside particles from getting to you), when 2 people wear masks that reduces the chances of transmission in a given retail encounter by 75%.
Reducing those odds by that much, when (from an epidemiological POV) the biggest math factor is to drive the r number down below 1, it’s a huge deal. If you do that consistently, the virus becomes rarer and rarer and has fewer opportunities to mutate and more importantly, you’re feeding fewer and fewer human beings to it.