Masks are for both reasons. I wear a mask while riding because often I do end up really close to people, either passing them or at a light, or just because of bad timing. I'm also breathing heavily while riding past a ton of people's driver side doors, and open car windows, and maybe there's a risk there? Of course if I stopped somewhere I'd definitely want to be masked, so may as well just wear it.

It's not hard to wear a mask, and I don't have the omniscience required to know if it would matter each day I wear it, so I try not to overthink it. Especially right now I think it's good to be a visible sign to others that you can be masked and do anything, so if I'm outside, I'm masked. If everyone followed that basic rule, and didn't try to rationalize how safe the way they're doing it is, we'd be over this already.