Regardless of the financial arrangements, interns are not the same as volunteers. Interns earn credit based on what they learn from the experience, not solely on a performance evaluation. They are accountable not only to a site supervisor, but to a faculty sponsor who evaluates the quality of the learning experience. A volunteer may perform the same tasks equally well, but may or may not be self-conscious about the learning derived. As interns, students must be self-conscious in order to earn credit.
Internships are, first of all, learning experiences.
Some people learn more effectively through experience. Internships provide that opportunity. The credit interns earn is based on what students can document they have learned, not solely on the evaluation of their performance at the site. Most students report working just as hard, if not harder, on internship as they do for classes on campus. Pursuing an off-campus internship along with several other classes creates a demanding schedule and requires a lot of time and energy.