The issue of masks has been discussed in other threads. I
posted a few studies on use of simple surgical masks vs N95 masks vs no masks before.
To summarize there have very few studies studying utility of masks. One from
China showed no benefit to a simple surgical mask in a hospital setting. The N95 masks did work
A
study from Canada in hospital nurses had both N95 and simple surgical masks equally effective or equally ineffective. There was no control group to see what the rate with no mask might have been.
Lastly a
study from Australia where there was a sick child with flu like illness in the home. Some were given no mask, some a simple surgical mask, some an N95 mask with no specific fitting and the rate of the adults becoming ill was compared. The wearing of any mask was calculated to reduce contagion by 60-80% with no difference in success with either mask over the other. There are lots of imperfections in this study including this was intense exposure, poor compliance with keeping the mask on, and exposure to the sick child before entry into the study. But there was clear evidence that a simple mask in an intense exposure was much better than nothing.
I could not find any studies on wearing a mask in a community setting. But it makes sense to me that some barrier is better than no barrier. Whether the barrier works by keeping droplets from being inhaled or just as a reminder to not touch your face, or scares strangers away from you, whatever the mechanism. When the CDC says there is no evidence that wearing a mask will protect you from Covid in a community setting, that is 100% true. However the reverse is also true. There is no evidence that it won't help.
The CDC is correctly trying to advise on evidence of what does help.. hand washing, proven, avoid touching your face, strongly suggestive proof based on mode of entry, spacing based on knowledge of droplet mechanics. Mask.. no evidence or weak and contrary studies. But those who say it can't hurt, other than depriving medical personnel of PPE [a big point not to be ignored] have some merit in their argument.