Here's an excellent graphic highlighting the difference wearing a face mask can make in combatting the spread of the Coronavirus.
Figure 1 - the risk is at its highest if no one wears a face mask.
Figure 2 - the risk falls, but is still high, if the only person wearing a face mask is the one who is not infected.
Figure 3 - the risk falls again if the person carrying the virus wears a face mask.
Figure 4 - the risk falls to its lowest if everyone wears a face mask when social distancing is more difficult.
Since no one knows for sure who has the virus and who doesn't requiring people to wear face masks in more confined spaces can play an important role in stopping the spread of COVID-19.