Definition:
One rough and ready way to define the term "science fiction" is: "That branch of literature which deals with the response of human beings to changes in science and technology." (Isaac Asimov)
SF as an art form:
As a type of literature, and thus as an art form, I do not believe science fiction needs any other justification for its existence. Similarly, I do not think Raphael's "School of Athens" painting requires any other justification than that it exists. It is beautiful, it is true, it is one of the triumphs of humanity. Some of science fiction is not very artistic, but I think the best of it is.
Other benefits of SF:
Even aside from artistic values, science fiction has many salutary effects upon society. Some of these include:

1)Science fiction promotes interest in science. In a world that is increasingly oriented to science and technology, this is very important. The general public's knowledge of science is abysmal: test scores show that American students' scientific knowledge is far below that of students in several other industrial countries; the percentage of American students majoring in science and technology in colleges is far below that of Germany and Japan. When scientists are asked what led them into their profession, many cite science fiction as a key inspiration in their youth and beyond.

2)Science fiction promotes learning. Many teachers use science fiction to spark students' interest in the world around them. And because SF is based on science, the stories often enhance students' understanding of scientific ideas.

3)Science fiction enhances understanding of the processes of sociology and history.
Since science fiction deals with the responses of society to changes in science, many of the stories contain much sociological content. It is an ideal way to examine our culture by contrasting it with alternatives. For example, Brad Linaweaver's Moon of Ice is an alternate history wherein Hitler won World War II. In it the reader is forced to consider how this change might have affected our world, and thus it becomes clearer to the reader how complex are the processes of cause and effect in history. It is "history in reverse."

4)Science fiction reduces biases in our thinking. By opening our minds to all possible changes, we are forced to examine the validity of our own beliefs. By reading Ursula K. LeGuin's The Left Hand of Darkness, about a society where a person's sex constantly shifts from neuter to male to female and back, it becomes apparent to the reader how our own sexual mores are largely determined by our particular circumstances, and are not necessarily the absolutes we often take them to be. It is no accident that the TV series "Star Trek," which portrayed a future society wherein all races of humanity cooperate peacefully, came during the civil rights era of the 1960s. It promoted the radical idea of racial equality. And if you could accept the alien on board the ship, how could you not accept another human being of any skin color or sex?

5)Science fiction is a reflection of our culture. This is true of all art forms, and is an ongoing process that helps us to reflect upon the world we live in. Any society without art is a dead one.

6)Science fiction creates a sense of wonder with the universe. A science fiction fan can never be bored with the world.

7)Science fiction forces us to think about the future. This, in my opinion, is its most important function. In a nutshell, sci-fi readers are thinking about the future. Whatcould be more important? Some of those futures we'd like to avoid. Ray Bradbury's Fahrenheit 451, describing a future America where knowledge is so feared that all books are burned, is an eternal warning against tyranny and ignorance. The Drowned World by J.G. Ballard infuses in the reader a determination not to let the world's environment be destroyed. Stanislaw Lem's The Futurological Congress examines the frightening probability that science will evolve to the point where governments can control people's perceptions of reality.

Science fiction takes the lessons of history and creatively projects them into the future (usually) to allow us to examine the consequences of our actions today. As an art form, it does so by combining ideas with human(or alien) psychology to show us the very real consequences of a changed society. We can feel it, and we respond to it. It is no accident that science fiction readers are also usually environmentalists, for example. They want a better future, and they want to start creating it today.