The mask had two small nose holes and was a type of respirator which contained aromatic items. The beak could hold dried flowers (including roses and carnations), herbs (including mint), spices, camphor, or a vinegar sponge. The purpose of the mask was to keep away bad smells, which were thought to be the principal cause of the disease in the miasma theory of infection, before it was disproved by germ theory. Doctors believed the herbs would counter the "evil" smells of the plague and prevent them from becoming infected.
Actually might help avoid some air borne infections...