The 3d sublevel is not filled until after the 4s sublevel, because the 3d sublevel has more energy than the 4s sublevel, and less energy than the 4p sublevel. (:
They reduce viral transmission (if worn correctly). ...
They prevent asymptomatic spread. ...
You're protecting others from illness. ...
They're mandated by law in some states. ...
They're good hygiene in general.
Sailors sail at different latitudes when they travel East or West because of the direction of the prevailing winds varies with latitudes.