Not sure about America specifically, but there are various reasons why people do this in east asian countries:

• The person is ill and wants to contain their illness (almost totally ineffective due to the vast majority of people not following hand-based hygiene, but a visible sign that you are trying, so therefore popular)

• The person is not ill and doesn't want to get ill (again, without very good hygiene around frequent hand-washing this is mostly a placebo effect)

• The person has hay fever (if worn with a good fit this can filter out pollen reasonably effectively; really bad sufferers also sometimes wear close-fitting glasses/goggles)

• The person is trying to avoid their mouth/nose/throat drying out, as East Asia has much less moisture in the air than Europe during autumn, winter and spring; not sure about vs. the US though (seems reasonably plausible, but again will only work if the mask fits properly)

• In china, masks are extremely common in the main cities due to the pervasive air pollution; perhaps they want to avoid smog?

• The person wants to be relatively anonymous or keep their distance from others (as an English teacher in Japan I often ask students to take off their masks as I need to see their mouths sometimes; I also think it's bloody antisocial. I note that many of the people who wear masks to class are the shyer ones).

In general the idea of the mask is much more important than the actual practical usage of it. I say this because a lot of the people I see don't seem to have worked out how to use them properly. Here, you often see people with the mask only covering their mouth and not their nose, or even with it pulled down under their chin. Some older people wear masks that are made of cotton and have such large fibres and gaps between them, and which fit so loosely that they will have zero effectiveness. Many masks contain a strip of metal on the top edge and the idea is to bend it to fit the bridge of the nose and seal it against the face, but the majority don't bother. The really important thing, it seems, is that you wear one to show your consideration for others, irrespective of the fact it's completely pointless.