The clean fit to a face is pertinent to zero exposure. This is a common test in the military: dawn your mask, cover the vents, and suck in. If you don't feel immense pressure on your face, if you can't shake your head and do it again then you're metaphorically dead. Part of this fit is having a mask that fits your face and being clean shaven.
I don't know that COVID protection needs to hit this mark, since this is designed for nerve agents and nuclear fallout. It seems like trapping quantities of the virus is good enough, since it takes some threshold of exposure to get someone infected.
https://link.springer.com/article/10.1007/BF01658736
https://ncbi.nlm.nih.gov/pmc/articles/PMC2493952/pdf/annrcse...
for surprising null or even slightly negative result of surgeons masking vs. surgery patient wound infections.
Assuming linear scaling or other naive-model benefits from masks runs into large-scale gen-pop experience, which has now been studied by enough RCTs to cast doubt on the assumption. Long-term WHO and CDC policy guidance was based on empirical science showing non-pharmaceutical interventions are not very effective against respiratory viruses.
The fact that others are wearing surgical or even FFP2 masks is a protective bonus for me, even if the former vent or it doesn't protect them in all cases. I believe this was the reasoning of the German original fabric mask mandate starting spring/summer 2020: mask + no mask = 0, but mask + mask = good protection against droplets. Whether that made any significant difference with the transmission in aggregate is probably impossible to tell now, but it probably did prevent at least some infections. More importantly it made masks more normal, compared to spring where you were looked at like a maniac for wearing one.
Regarding the graph, Germany was and still is in a lockdown, so numbers will be going down masks or not. Upgrading the mandate to surgical/FFP2 was done more out of fear of the new mutations and it wasn't FFP2 only because it's economically / socially not tenable. I don't know why the US decreased, but the US is way ahead on vaccines and also had more infected, so I assume it will continue to do so.
Germany and USA: https://twitter.com/ianmSC/status/1359677558829191169