Suppose person is coming down with a cold or Covid.

Suppose the person sneezes or coughs.

If the person has a mask on, the mask will slow down expelled droplets and even absorb some of them. The droplets will not go as far and will not contaminate as many surfaces.

Even a person who has not coughed or sneezed observes that the mask gets wet from normal speech. Therefore, the mask is stopping droplets.

How can you argue with that?