Daniel Siedell offers an answer:
A painting is a weak and vulnerable thing because it is just not necessary. Smelly oil paint smeared across a canvas cannot be justified in this conditional, transactional world. Yet vast, complex institutions and networks have emerged to do just that, whether through the auction house (art as priceless luxury item), the museum tour (education), or the local chamber of commerce (art as community service, cultural tourism, or urban revival). That art is ultimately gratuitous, that its existence is a gift to the world, creates anxiety and insecurity in the art world. Everyone involved, from art collectors and dealers to critics and curators have to justify their interest in this seemingly “useless” activity—and justify the money they make or spend on its behalf. Art simply cannot be justified.
He argues that, in response, "many artists cultivate a certain kind of behavior—craft a social role—that simultaneously justifies and protects their work, offering a marker for art collectors, curators, dealers, and critics, while releasing them of the burden to have to explain or defend each work they produce."