The request was in case you HAD the virus.
Which is largely ... stupid.
If you have the virus, you should be self-quarantining (or quarantined in hospital if you have serious symptoms).
In your house, you should wear a mask to protect family member/caregivers.
But you definitely shouldn't be out in public, mask or no.
The other problems with that advice: Many folks will have only mild symptoms and not know they have the virus. Symptoms are similar to cold and flu. Test kits aren't readily available yet (a month plus into a pandemic!) so it's hard to know who actually has it or not. So best if anyone with a cough, or any sickness wear a mask in public.
Masks are imperfect, but a reasonable precaution and help stop the spread of disease, by largely keeping your germs with you, blocking others germs and reducing the instances of touching your nose and mouth with potentially contaminated hands.
The fricking USofA can't produce enough simple face masks to protect the country and anyone who feels the need to wear one. That's pathetic. Again, where was the preparation? Shanghai, maybe 1000 miles from Wuhan has been on near lockdown for over 5 weeks. Didn't the Trump Admin make any plans during that time. Like stockpile and produce more facemasks. hell, it could be the one bright spot in the economy along with wet wipes and hand sanitizer.