It does look kind of funny seeing someone wearing a mask outdoors far away from anyone else. Sometimes it's just easier to keep it on, rather than taking in on and off all the time; especially when riding a bicycle.
Sometimes, wearing a mask is just to not rock the boat around other other people who shutter at the sight of a mask less person passing them. I almost always wear mine when in crowded areas outdoors and, of course, for indoor public spaces.
When I bike pass people on trails, I sometimes hold my breath; especially if I don't have my mask on. That might work as well as the mask, if not better, in short passing. I wouldn't know. One seldom hears anyone talk about briefly holding one's breath. I would think this can work walking past people as well.
I usually don't wear my mask while biking on roads. Especially on long rides, or going up hills, the mask can get wet and become (I think) less effective. It also slows down breathing, but I could live with that, if I had to, by just riding slower. I don't ride real fast anyway.
It's hard to keep taking it off and putting it back on. I bike back and forth from streets, where everyone is in cars, to trails. For this reason, I often just leave it on. I usually leave it on downtown.
Some people have the mask around their chins and pull them up when passing a person outside. I find that my mask falls off if I just keep it around the chin. Might have something to do with my bike helmet straps. I usually have it in my pocket. Not as handy to pull up though.