if you look up "does wearing a mask" on google, you'll find dozens of articles discussing the mixed messaging, how some countries encourage it while others discourage it, etc. so there's tons of back and forth with reasons/studies cited on both sides.

I've yet to see a definitive answer but essentially, one of the big arguments is that if you don't know that you have COVID-19 and since it's asymptomatic when you're already infectious, wearing a mask in public may help prevent the spread to others.