They are supervised by those trained in the profession. I was a junior reporter assigned to cover environmental issues in Florida. My desk was between the bureau chief's office and one of the reporter's cubicles. It was a great place to be for four months - I was able to learn as I listened. Listening to the events in the newsroom taught me more than any class ever could.
Classes are important and have their place.
They are vital to learning skills and techniques, but an internship allows you to put those skills into practice. An internship lets students know beforehand if their major is something they want to be doing after graduation. Internships can also provide opportunities for future employment. Many companies are more willing to hire their interns than those who are new to the company. This practice allows the company to already be familiar with the applicant's skills and pe rsonality.