Art
"All art is quite useless," said Oscar Wilde. I call bullshit.
Aside from considerations of art as beautiful, fun, and a communication medium (all of which are, I think, important uses for art), there is a fundamental need for art in society. Art allows us a space for imaginative exercise. Reading a book, looking at a painting, watching a movie, any sort of engagement with art (active, not passive, engagement) is mental and, more importantly, imaginative exercise.
The imagination is important because it is our imagination that allows us to identify with other people. If we can't imaginatively inhabit a space where we can understand how other people feel, then we cannot sympathize or empathize with another person. If we are unable to do these things, then we have nothing upon which to base the social contract that considers other people worthy of consideration as feeling, thinking individuals outside of ourselves. If we can't understand that other people have feelings like our own, not just know this intellectually but understand it on an emotional level, then we can be nothing more than sociopaths, acting only in our own selfish interests for instant self-gratification. If we do not engage with art, then we cannot develop and maintain the imaginative power necessary to connect emotionally with other people, and we cannot have a civil society.
Put that in your pipe and smoke it, Mr Wilde.