It’s no secret that ear-loop masks are a common sight in Japan (and Asia in general, really). Ostensibly, they’re meant to keep your germs in or others’ germs out, but that’s not their only use. Besides the above, people wear them when…
… they’re trying to hide a zit or other blemish;
… they didn’t feel like wearing makeup (girls only);
… it’s cold and they don’t own/want to wear a scarf;
… if the Love Live! anime is to be believed, they want to hide their identity (requires hat and dark glasses as well).
Personally, I have no desire to wear one of the damn things. They seem confining and bothersome. I’ll suffer my colds and risk sickness here as I always have: barefaced.