It won't help with a new virus... but it will help with the old ones. People die all time from various diseases they caught from other people, but it's not a pandemic so we don't worry about it. Lives can be saved if we knew who is vaccinated, who is immune, and who is a carrier for a range of potentially-fatal infectious diseases.
I spoke once to a lady that worked at a medical centre treating refugees and asylum seekers. Some were diagnosed with TB, were told to always wear a face mask when in public places, and attend regular monthly follow-ups. Few wore the mask, and all disappeared without trace withing a few weeks, never to be seen again, probably living in the community to this day.
PS - this is not an anti-immigration rant, it is just when people have an infectious disease and no fixed abode, this combination presents a potential public health issue.