Masks are important as part of an overall strategy - ie they’re one part of the jigsaw. Here’s the advice from WHO:
“If COVID-19 is spreading in your community, stay safe by taking some simple precautions, such as physical distancing, wearing a mask, keeping rooms well ventilated, avoiding crowds, cleaning your hands, and coughing into a bent elbow or tissue. Check local advice where you live and work. Do it all!
Make wearing a mask a normal part of being around other people.
Masks should be used as part of a comprehensive strategy of measures to suppress transmission and save lives; the use of a mask alone is not sufficient to provide an adequate level of protection against COVID-19”
@helli is quite right about conditions that make someone exempt from mask-wearing - and about psychopaths and narcissists too. It’s a small thing we can do to help reduce the risk to other people. There’s plenty of evidence online about how effective various masks are. They’re not a magic answer and might only be a small part of the solution, but every little helps, as they say.