The whole concept behind wearing a face mask is to minimize the risk of the transmission of coronavirus from asymptomatic carriers. Additionally, it may also act a barrier in a close space, where people are coughing, sneezing, laughing or talking. It may prevent immediate viral exposure but the effectivity depends on how it is worn.
A lot of people wear face masks incorrectly, which eventually renders them ineffective. If you pull the mask down to talk, wear it below your nose or even keep touching the outer layer of the mask with your hands and then touch your face, it completely negates the benefits of wearing it. Secondly, there is no clear directive by medical experts on the re-usage of face mask, even though cloth and home-made face coverings may be washed and used again.
Additionally, the availability of face masks remains a big issue, as medical staff including doctors and nurses are battling with acute shortage of the same in the middle of a pandemic.