The idea is that the masks filter particulates at the source, and then again at the destination. See https://www.livescience.com/face-mask-visualization-droplets... for a visualization. Reasonable. Wearing masks when having symptoms is elementary courtesy, and self quarantining is even better. Unfortunately we've embraced the 'asymptomatic spreader' idea a bit too much and now we regard anyone that doesn't take the most extreme measures at all times as a de facto murderer, because there might be a tiny chance they are infectious. Not healthy.