That if you have symptoms, you need to wear a mask.
But at least 50% of people are believed to be asymptomatic. Asymptomatic people emit somewhat less virus than people showing symptoms, but they are still emitting a lot. It's believed that most new cases are actually being caused by asymptomatic people.
In addition, masks also do appear to protect the wearer to an extent--see the links. In mask-wearing societies, mask use is imperfect but does appear to provide some protection.
In particular, it provides an immediate physical barrier if someone sneezes or coughs on you, which will deliver a big dose of the virus.
Getting exposed to a lot of the virus at once (being sneezed or coughed on, for example, as opposed to "touch a surface, touch your face" which will deliver far smaller amounts of the virus) probably increases the risk of developing severe symptoms or dying, as the immune system has less time to mount a defense, this is what I have been told.
High viral loads are probably the reason why we see more deaths than expected among quite young healthcare workers---they get exposed to gobs of mucus etc. when doing things like intubating patients.
Would be interesting if some regions in the UK took the initiative and rolled out mass mask-wearing, as it could enable us to see how much difference this makes.
In the meantime, even putting a scarf over your face may be better than nothing.