If you're like most people, you work for money. It's great if you do what you like and not just what you know how to do, but you have to pay the bills, so if the paycheck wasn't there, neither would you. However, before you have mortgage, car payments and a family depending on your salary, in times when you're still in school getting help from your parents or a scholarship and making ends meet with a part time job, trying a dream job, even if it is for free, may be the best way to determine if it truly is your dream.
No matter if you will or will not get paid, you should consider applying for an internship as soon as you're eligible, as it will give you an opportunity to see if you've made the right decision picking your future career. Spending part of the summer in a New York biotechnology laboratory or a web design Miami firm will give you the insight necessary to decide if you want to follow through or maybe change your career path, so it's worth sacrificing several weeks of sun if tat way you will know for sure you're not wasting your time in school.
For those lucky to be sure of the career choice, a well chosen internship is beneficial on more than one platform. For one, you will get a hands-on experience, even if you only get to do seemingly unimportant things, in the field that you want to spend your working life in, which means while others only learn from books and controlled school practice, you learn from professionals in your field; and two, before you even get on the job market, you'll already have resume worthy experience, which may open before you doors that are typically reserved for seasoned workers only.
During an internship you learn to use your school knowledge in a real work environment, but you also learn what you don't know yet. If you're doing well in school you start thinking there isn't much more you can learn from your teachers, when in fact school knowledge is just a portion of what you'll need to know before having a chance at professional success. That's why you have to note what you still need to learn and ask to shadow actual employees during your internship as well as talk to your instructors upon returning to school whether it's possible for them to help you out.
In real life most people can't afford to work for free. As an intern you seemingly do just that, but in fact you are working for knowledge and experience, which at this point in your life is the best payment you could ask for. If you chose a paid summer job having nothing to do with your education over an unpaid internship, you could buy for example a new pen tablet, but it's much more valuable for you to learn new ways of utilizing that tablet than just having it sit on your desk.