Because it DECREASES the chances of any one individual spreading it It's not a simple black and white you either spread it or you don't.
If you're vaccinated you're less likely to spread it than someone that isn't. You also are less likely to have severe symptoms. This is a good thing. As pointed out this kind of logic is the same with Flu vaccine... and this is saving a lot of people every year and it's only "50% to 60% effective" (using that article above as the basis).
