If you are asking why humans need it for our basic survival . . . we don't.
Of course I don't think that is what you meant by your question. I'm sorry to say that there is not one, right answer to this question. It's going to absolutely depend on who you ask. I'm not an art guy. I never have been, but that doesn't mean I don't see its value.
Art is valuable to a lot of people because it is a way to express their emotional state. It gives a physical representation of an abstract feeling. Because of that, some people need art because it helps them release joy, anger, sorrow, pleasure, etc. Other times the art functions best because it is more effective than words might be.
I have friends that are quite artistic, and they have told me that it helps to clear their head. That makes a lot of sense to me. That's why I run. It helps clear my head, but to a non-runner that sounds crazy.
People need art because it can be used to teach. For example, reading a description of pain and suffering in the dictionary doesn't quite capture the sentiment that a photograph of starving war victims does. We need art, because it's so much more than pictures on paper and statues in parks.
I could write an essay on this question but I will try to keep it short.
While we do not need art to survive, art has given us the gift of life. With art we can see into the past--what people looked like, maps of ancient cities, what people wore, how they created music, and so much more. Art allows us to express ourselves when words may not be enough. Art shows us how we have evolved as humans--from people who had language and used art to communicate hunts, to poems, songs and stories to help us learn about the past and what is coming in our future.