No matter what your preferred art medium is, it's unlikely that you will be able to afford fancy equipment at the beginning of your career. Most art schools have invested in expensive computer, visual art and camera equipment. As a student, you typically have access to this equipment in a supervised environment. Learning to work state-of-the-art equipment is important to working in the art world, and this will give you a leg up on those who have little experience with such equipment.
Sometimes people without formal art education dive headfirst into an artistic career without prior experience or knowledge. Students may be able to develop their skills adequately without a teacher, but they lack the foundations necessary to expand on their skills. Art school teaches students both the basics of their craft as well as the history of their art of choice.
Going to art school puts you in touch with other like-minded and creative students as well as teachers who have experience in their fields. Because of this, you will be able to form art collaboratives and find people to work with on projects when you graduate or perhaps even a future business partner. Teachers not only can guide you, but likely have connections in the art world that give you access to internships and other career opportunities.
Many art schools require that students complete an internship before graduating. Because of this requirement, schools often form lasting relationships with prestigious companies, allowing their students first pick at many job sites. Working for a prestigious art company will allow students to make further outside connections in addition to exposing them to the working world.