If you are near someone in a risk group it seems very good advice to use or even mandate the usage of masks. Same for being around high risk areas such as using any mass transportation. For low risk areas the benefits are less clear and the potential downsides seems to have about as much evidence as the benefits.
But we don't. Because it'd be inconvenient. Safety is not anyone's highest goal.
If you are hanging around and talking to someone at a fairly close distance, masks simply won't do anything other than visually signal distance is required to others. And, in the unlikely event someone sneezes or coughs, it probably prevents widespread contamination of the surrounding area.
The Seattle choir episode and some studies out of China in restaurant air handling systems seems to indicate the virus is airborne and can be spread to others within a fairly large area, with time and proper airflow. This will happen with or without a mask in place, in my opinion.
I'm super nervous about sharing indoor space with people I don't know, so I don't. The more time we can buy ourselves, the better the outcome.
A Google search result for droplet sizes: https://www.pnas.org/content/115/10/E2386
A Google search result for effectiveness of various masks related to particle size: https://blogs.cdc.gov/niosh-science-blog/2009/10/14/n95/
People seem to have a huge misunderstanding about how viruses travel through air (which is how you get stupid memes like this[1]). There aren't isolated viruses floating around passing through spaces 1/10th of a micron or larger. They need a medium of which to travel upon (such as saliva droplets) that are much larger than the virus itself[2] (usually 5-10 microns)
1. https://starecat.com/content/wp-content/uploads/im-wearing-t...
2. https://www.npr.org/sections/goatsandsoda/2020/04/03/8256393...
Example: measles is spread airborne and the R value is > 10. Coronavirus R value is < 3
Bad ventilation and dry air keeps the airosols floating around making it easier for people to inhale them. The spread in meat factories is one example.
But a mask still can prevent a lot of airosols. While also being limited. There was research that shows a lot of air escapes above and below the mask if someone coughs or sneezes.
It's not (for the vast majority of people the vast majority of the time) but the benefit is to other people not you and well acting in other peoples self interest seems to be seen as weakness in parts of the world (including sadly mine).
FWIW I always wear either a mask or a bandana over my mouth/nose when I'm out and about these days not because I expect it to protect me but because it might protect someone else.
I expect my opinion is swayed by my best friend having Cystic Fibrosis for which Covid19 is essentially a death sentence and a mother who has chronic health issues.
From my perspective alongside all the other advice wearing a mask seems sensible.
Same reason I wear heavy duty motorcycle gear and a helmet when I ride my motorbike - I don't plan for the crash but as the saying goes, you don't wear it for the ride, you wear it for the slide.