I heard an interesting analogy today - which makes it clear why everyone wearing masks makes sense :
Imagine we are all walking around naked from the waist down - eeurghh - and the bloke next to you can't hold it in any longer and has an unexpected wee - eeurgh again.
If you are in the firing line your legs will get wet.
But ... if you are wearing trousers they may reduce your wetness just a bit.
But ... But ... if he is wearing trousers the stream will be largely contained and, if any does escape through, then your own trousers may just sop it up before it reaches your skin.
So - we wear masks for two reasons :
1. To protect others from our emanations (the main reason)
2. To protect ourselves just a bit (a minor reason, but still possibly useful)