Common sense says that any kind of barrier between cough or sneeze droplets and my nose or mouth will reduce the probability of me breathing it in. I could tie a scarf around my mouth. It may not be perfect but even if it reduces transmission by 5%, Why not?

https://smartairfilters.com/en/blog/diy-homemade-mask-protec...

Has more info on studies related to this. I believe their claim is that tying a cloth around your nose/mouth is 50% as effective as a proper mask.