Mostly for dust. A lot of the smoke/dust particulate matter from the 9/11 attacks has been linked to nasty cancers, even among civilians simply fleeing from ground zero.

Most N95 masks are for construction to deal with dust and harmful particles from insulation and other materials. These masks are noish approved and not FDA.

I am curious what construction workers used in lieu of them. Cloth masks isn’t going to give you the protection of harmful dust from finishing a countertop or doing wood work.

Also for forest fires - the smoke.