It works both ways. A 25% reduction (or whatever positive number you agree with) is better than 0%. Very simple math.
Despite all the pleas and demands, it is not arguable that masks have done little if anything. There is not one state or city, that by having the population wears masks, has shown any drop in infection. Not one. And it seems quite obvious that by reusing a mask you are simply re-exposing yourself to whatever you walked through earlier.
I may walk through a room with virus particles and get a few inhaled. But when I leave the room, I leave it behind. A mask wearing person under the same conditions takes the virus with them and unknowingly exposes themselves over and over.