What does it mean for art to be ‘useful’ ?

While searching to define the term useful for my essay, I found a website that defines the exact opposite of what I was thinking.

http://www.weareuseful.com/about/ :

It is all about using art to promote fashion, commercial brands and products through advertising.

A real example that art can be useful for the fashion industry and the capitalist market.

But can it be used against it too?