To learn specific skills that are hard to learn other places. There are some great schools that really focus on today's new music environment (Berklee and Oberlin come to mind), as opposed to the more traditional schools. Learning skills like music production will put you miles ahead of the throngs of people using software like Garage Band at home. Music school actually teaches some really important life skills (I'm serious), and graduate programs from medicine to law look at music school majors who were serious about their craft as people who know dedication and time management. This is surprising but true. Trust me, I know about this one first hand. Don't get me wrong, you still need to have all the requirements for whatever field you are leaving professional music for, but your music background makes you perceived as "interesting and well-rounded."