A growing number of studies point to the mask as a powerful tool that can help control the spread of the coronavirus. The primary way the coronavirus spreads is from person to person by respiratory droplets produced when an infected person coughs, sneezes or talks. Face masks, however, can block these droplets. 

Lots of people are wearing face coverings that offer essentially no protection against the virus. You should be wearing a surgical or N95 type mask or a DIY mask made of many layers of high thread count fabric, or non-woven fabric. Any mask is better than nothing. Because even an N95 mask that filters out 95% of virus sized particles or more still lets 5% (or fewer) through, and people who think "face masks stop Covid" go out into contaminated areas when they don’t absolutely have to instead of treating the mask as what it is, the last resort. Face masks and other precautions lower risk, they don’t magically eliminate it.

Because lots and lots of people are selfish, ignorant bozos with an overblown sense of self-worth. Even if a cheap mask is only 90% effective, it would reduce transmission by 99% if everyone wore one.

SARS-Cov-2 is extremely contagious. Although a mask can reduce your exposure to virus particles by something like 95%, that remaining 5 % can still make you sick. If you ONLY go around others when you absolutely have to and then diligently wear a mask, your innate immunity (your immune response to any foreign matter in your body) may be able to destroy any virus particles that get through before they can multiply. But the more you go out, the more your roll the dice, giving Covid another chance to overwhelm your defenses. Even the best mask is, at best, only about 18% effective if worn like this:

Since SARS-Cov-2 can infect most tissue types in the body, it can bypass the mask and go in through your eyes. That’s less likely because your eyes aren’t actively sucking it in like your lungs, but it can happen.

Ditto the digestive tracts, although hot food in a closed container should quickly inactivate the virus, cold food and the container itself, will not.

A mask can help—but if you keep futzing with it, you can smear the virus around underneath it. If you get the virus on your fingers and touch your mouth, nose, or eyes, it can infect you that way.