I would think they are a weirdo.
Only time I ever saw this was at a trade show in Texas about 15-16 years ago when there was a huge flu epidemic, and a few Asians visited our booth wearing the masks. Its not something Id do, but the flu was so bad that year I couldn’t blame or laugh at them.
As parent of a former pediatric leukemia patient, I would presume it is to protect the fragile immune system of the person wearing the mask.