At one point, it was determined that wearing a cloth mask over a surgical mask afforded the best protection.
It would be kind of pointless for a healthy person who tested negative to wear a cloth mask. Others who were infected wearing cloth masks would protect the uninfected
However, if everyone wore a minimum of a cloth mask, this would have greatly reduced transmission. It made more sense to just tell everyone to wear one than to say only those infected and contagious must wear it.
Fauci should have been setting an example by wearing one in public even though he was not infected and contagious.
It's better to have it and not need it, than need and not have it.
It's not that hard
The virus attaches to droplets, large and small. Large droplets exhaled by a masked person will be stopped. Droplets small enough to pass thru the mask will go thru and keep on going. How far a droplet travels depends on its weight.
An elementary school nurse in Podunk, Kansas may describe it as ' get some chicken wire and try to throw a beach ball thru it. Now get a pea and see if you have more luck.' Then get 2 pieces of chicken wire and align them to make the gaps smaller and do the same.
After passing thru the chicken wire the large peas fall to the floor sooner than the small peas due to gravity.
That's essentially Fauci's position on masks. When it was determined that the concerning mode of transmission was not large droplets but aerosolized droplets the guidance changed. It's not groundbreaking rocket surgery that Fauci came up with.
But yeah, let's give Fauci credit for telling everyone to wear masks because of the work he was funding in BSL-2 labs in a communist country that we can't trust or get information from.