If you're asymptomatic, you won't get Covid or even know that you have it, but you'll pass it on to someone else. Wearing a mask is good, even if you're vaccinated, if you're in close vicinity to someone else or a group of people like if you were on a bus or train with closed windows. Wearing a mask or getting vaccinated is also more to do with the safety of people around you - if you're absolutely immune or sure you'll never get Covid.