…in April, researchers at the University of Hong Kong and in Europe calculated that if 80 percent of a population can be persuaded to don masks, transmission levels would be cut to one-twelfth of what you’d have in a mask-less society. However, that study has yet to be peer-reviewed.
You could have Covid-19, not realize, cough, and give the disease to someone else. Best way to prevent that is to wear a damn mask.
Your personal actions have an affect on the public health. So, it needs to be regulated.
Vox has a fantastic video that dives into it more deeply.