So many students go to school eight hours per day in high school or take course loads of twelve to eighteen hours in college. Being in school for eight months of the year this much per day requires a lot of students. For each hour you spend in class, you are expected to spend at least three outside of the class doing work and studying material.
For many students, once you reach course loads of these proportions, having a job is almost impossible after adding extracurricular activities, a decent sleep schedule, and some kind of social life. During the summer, many students do not have these same kinds of obligations, so having a summer job is more feasible.
Despite what your teachers will tell you, for most professions, a large quantity of the material you learn in the classroom is not important for your adult profession. High school especially teaches you how to learn and work efficiently. College material is often more relevant to your profession, but college also teaches you how to think critically and evaluate problems. All of these skills are important, but they are not tangible. Summer jobs provide the opportunity for students to learn tangible skills like decorating a cake, changing a diaper, or tying knots for a boat. Summer jobs come in many shapes and sizes, so you could learn a variety of skills during the summer.
The real world is full of opportunities to use the non-tangible skills learned in college, but having a wide variety of practical and technical skills will make you a more valuable part of society. If you can speak another language, change a tire, and cook a meal for a large group, you will be more successful in and outside of the office one day. Acquiring skills that make you a well-rounded individual will always help further your opportunities.