Many masks rely on a good seal against the face so that, when you breathe air in, it is drawn into the filter material where the air is cleaned. If there are any gaps around the edges of the mask, ‘dirty’ air will pass through these gaps and into your lungs. It is therefore very important that you put your mask on correctly and check for a good fit every time.
Facial hair – stubble and beards – make it impossible to get a good seal of the mask to the face.
If you are clean-shaven when wearing tight-fitting masks (ie those which rely on a good seal to the face), this will help prevent leakage of contaminated air around the edges of the mask and into your lungs. You will therefore be breathing in clean air, which will help you stay healthy.
If there are good reasons for having a beard (eg for religious reasons), alternative forms of RPE, that do not rely on a tight fit to the face, are available.