In the UK it is now *compulsory* to wear a non-medical face covering when traveling on public transport, and in England this also applies to most public indoor spaces such as shops, banks, hair salons, transports hubs, and visitor attractions among other places. Please bear in mind that rules in England, Scotland, Wales, and Northern Ireland currently vary in some respects so it's important to check the guidelines for where you are specifically.
These guidelines are in place primarily to protect people around you from droplets you might expel through breathing, talking, or coughing; even people without symptoms may still be at risk of spreading the virus. The masks don't have to be anything fancy and are not the same as medical-grade PPE —the primary functions are that they are breathable, cover your nose and mouth, and are tight enough that they don't leave loose space for droplets to escape.