By wearing a mask you are protecting others (especially vulnerable people) from contracting Covid-19.
As we now know, the coronavirus can spread through droplets and particles released into the air by speaking, singing, coughing or sneezing — a mask helps prevent these droplets from infecting others. It can also protect you from being infected. This article from Johns Hopkins also answers FAQ's on other face coverings like buffs or bandanas, and also face shields, read it here