I've heard pro-mask people say various things like "even if it reduces it a couple of percent then it is worth it" or "it isn't so that you don't breathe the virus in, it is supposed to prevent you from breathing the virus out on other people."

What kind of logic is this? Especially the last one, which i hear / see often. SO let me get this straight, it CANT prevent me from breathing the virus in but somehow CAN prevent me from breathing it out? What sort of mask magic is this?