Now they are recommending wearing face shields in addition to a mask

Why?

Because coronavirus droplets might get into your eyes and infect you. By wearing a mask you prevent atomized droplets of coronavirus infecting you through your nose.

Wearing a face shield protects you from getting coronavirus droplets into your eyes and infecting you as well. This is why wearing a face shield is now recommended whenever you go shopping or work with other people in an enclosed space.

The first person I saw wearing a face shield was in a 7-11 type of convenience store. I remember thinking at the time that this was a good idea. However, she was wearing a face shield INSTEAD of a mask. But, now they are recommending both.

But, what if you have breathing problems or claustrophobia or other problems like Asthma or COPD?

What people with breathing problems could do is to wear their oxygenator while shopping or working which separates the oxygen out of the air for breathing.