(http://www.career.cornell.edu/students/findingAJob.html)
Internships or jobs – for a summer or longer – provide students opportunities to apply knowledge and skills to "real world" projects and to test career interests. Students learn how and what kind of work gets done in the world, how individual (and their own) skills and values inform work, and how to collaborate with colleagues of varying abilities and agenda. Such experience and knowledge are invaluable. They can recharge academic motivation and imagination. Your sons and daughters – our students – have usually been so successful in school that we sometimes forget they can be insecure about what happens after graduation.
In addition to providing personal confidence and a test of one’s actual interest in a particular kind of work, internships or jobs contribute to impressive resumes. Often, the students most successful in the job market have had considerable practical experience. The college’s, and the university’s, career services help students find internships and summer jobs.
No one, not even we humanists, would argue that learning is confined to the classroom. We all learn far more about the world from falling in (and out of) love, raising children, working in social structures, and in any number of other activities, than we learn in school proper. But we don't receive academic credit for such learning. The same is true of jobs and internships.