I'm very skeptical that my wearing a common mask protects me from anyone else. I do suspect that it reduces the possibility of me infecting someone else (hypothetically, assuming I'm asymptomatic).
Either a mask filters effectively, or it doesn't[1]. Intuitively I don't understand why a mask would only filter air in one direction.
[1] https://scholar.google.com/scholar?q=.%2C+ACS+Nano+10.1021%2...
I think of it like a device that is acting the way your hand does when you cover your mouth when you sneeze or cough. Your hand isn't filtering in any real way, but you are keeping droplets from flying as far as they otherwise would.
I think a good filter would require something much more like a WWII gas mask or a forced air respirator in a hazmat suit, etc...
Now, I'm sure someone will throw an obnoxious "citation needed" reply...
Does this constitute a lesser reason to wear a mask?
I do wear a mask when I go to the grocery store - I'm not an idiot. About 90% of people in the store also wear face coverings, even though they are not required. Our area is not in any way a hotspot, but I know I have a comorbidity (high blood pressure which I manage with ACE inhibitors), so I'm being extra cautious.
But I will take my money elsewhere if an establishment thinks it can "require" me to wear a mask, because unlike with the government, I don't have to be subject to their bullshit. So I haven't been to Costco or Home Depot in a while now, and haven't renewed my membership at Costco.