Masks of Bhairava, of which this is a very finely executed and luxuriously embellished example, play an important role during the Indrajatra festival. A stream of beer is released through a tube issuing from the mouth into a crowd of celebrants, eager to receive a few swallows. The beer or wine is considered a gift of the god Bhairava himself, who is a wrathful manifestation of Shiva.
Compare related examples in the Museé Guimet, see I. Alsop, "The Masks of the Newars," Orientations, September 1993, fig. 14, p. 57; and in the Zimmerman Family Collection, see P. Pal et. al., Art of the Himalayas, 1991, cat. no. 22.