I thought wearing masks was dumb until I lived in Japan.
Ideally, not going out into public and especially work when ill is best. But if you are going to, a mask does help somewhat from spreading your lergy onto others. It at least shows you somewhat care about the health of others.
Fuckers at my current work have given each other all kinds of shit from either coming in when ill or not wearing a mask when ill and breathing their miasmic breath over everyone.