Coronavirus...Why do we have opposite messages re using a mask. Some authorities say yes, some say no.?

I'm not sure that they really know the answer. 

It’s not recommended to keep virus out because virus are way too small to be trapped by the masks. It’s recommended to keep the virus from spreading to other people when you sneeze or cough. It’s especially help in keep it in because a sneeze is water droplets and mask can keep droplets from escaping. 

Ordinary mask is useless, is not designed to offer protection from dangerous microscopic viral particle.

Must buy respirator with filter.

a mask helps. but people who wear masks waste them by wearing them wrong and growing complacent with other facets of health like frequent hand washing. these people are more dangerous than they are useful.

on the other hand, we don't want them buying up the Mask Stocks when there isn't even enough for legitimate Medical Needs. obviously the masks help prevent infection and, equally importantly, prevent infecting others. if they didn't work Doctors wouldn't bother wearing them.