The reason the medics need such high quality protection, is that they are close to the patients for extended periods of time. The patient is coughing at them (I have heard the coughing of a Covid 19 patient and it is extreme) so they need that high level of protection.
For those of us that are at the supermarket, we are only passing by people who might have the disease (or ourselves) and with the correct social distance - the liklihood of infection is very much less. I would not be near an infected person without the full PPE.
Another reason for wearing a mask, I think, is that it helps remind others that there is a virus that is hugely damaging to humans.