They may not help that much with airborne diseases (as Covid turned out to be) but most diseases going around are droplet-borne. And, if they don’t help at all, why do medical personnel wear them?
Before covid, few did except for surgeons during procedures or dentists during procedures. Docs making rounds hardly ever wore masks in the before times.
Even back then there was no evidence masks help prevent the spread of flu or other respiratory diseases.
On one hand, I can see how it is easy to believe masks work -- it is very intuitive. But the history of medicine is littered with intuitive interventions that were later proven to be at best useless or at worst harmful.
So during surgery the doctor doesn’t sneeze or accidentally transfer bodily fluids into the open wound..
THAT is why they wear those masks.
Most of us have known that for decades..