The reason cases are going up right now is that in colder weather people spend more time indoors (poor ventilation, lack of sunlight and lower vitamin D, virus is also more robust since the strong sunlight in summer that "kills" it is too low in the sky to do this).

Some of the virus is airborne (and for much longer than just a few minutes). Microdust can stay suspended in the air for hours and drift a few miles. If you breath in just a small amount of virus, you likely will just get mildly sick or not sick. If you breath in a lot of virus, you could get severe pneumonia and other complications that require hospitalization.

A face covering in a public enclosed building (especially in winter), should block enough of the virus (actually it hitchhikes onto microdust) to prevent severe infection. Too much virus entering your lungs would probably overwhelm your immune system. It would be wise to wear a face covering this winter when in a public enclosed building.