As any professional musician will tell you, a career in music is more than just artistry and performance; the business side of music is a crucial component of a successful career in the arts.
The business side of things may include managing gig contracts, paying estimated taxes, sending studio invoices, setting rates, sending emails, professional networking, and attending conferences.
Many professional musicians do more than one thing - performing, teaching, gigging, writing, etc. You are your own small business and tending to the things that keep your business running is an important part of building a successful career. You don't need a degree in business to manage everything I outlined above, but you should take steps to be informed.
There are lots of helpful books, articles, and blogs out there to help get you started. Here are a few of my recent favorites: