There is a reason that 2.5 billion oriental people are wearing them when they have to go out. As Droughtquake and Zombie have noted, they do have their limitations but they are better than nothing when you have to get out to get groceries or to the drug store. Additionally, if they are washed completely with soap and water after each time you go out and are completely dry before wearing out again, they do prevent some spread of the contagion. Every little bit helps and that is why some places are making them mandatory.
The most alarming facts I've found about this virus is that scientist are unable to get a clue as to why some patients clearly infected are completely or nearly symptom free, while others have ongoing mild to moderate symptoms for some time, while others quickly deteriorate and need a ventilator in a matter of just days. They are still sorting out the extraneous variables and of course underlying conditions are a factor, but even some people with serious underlying factors don't do too bad and some fairly young people with no underlying conditions have devastatingly mortal effects. The health officials are less than transparent about the rate of death once a patient is put on a ventilator and how many recover and can get off. Finally the anecdotal details of those who survived and were weaned off the ventilator indicates all have some residual loss of lung capacity and most have some significant loss.
I'm not alone in thinking we know less than we need to know and until we have widespread testing for active virus as well as widespread testing for antibodies the epidemiologists and infection control specialists are just guessing as to when we can begin ending sheltering in place. Any vaccine, no matter how much money is spent is at best a year away and probably 18 months until one is widely available. I worry by that time the toll may be in the tens of millions....
My other worry is that as governments become more desperate in trying to control the epidemic, people are losing their liberties in unprecedented ways and they may never be reclaimed...