Competitive careers, like those in marketing and sales or design, often expect internships on their prospective new hires' resumes. An internship can help you get experience in your prospective professional area, even if the position isn't exactly what you had in mind or directly related to what you want to do. As long as you have work experience in a professional environment that teaches you some of the skills necessary for your job, you should be able to advance later on in your career.
An internship can provide valuable contacts in the industry you enter. For example, if you're in a graphic design degree program, an internship with a design firm can position you to connect with in-field practitioners, thus creating great future employment opportunities. Managers and coworkers who you have worked with during your internship can offer you advice and show you what it's really like to work in a given industry.
Additionally, many companies offer future careers for their interns. They could set you up with a position right after you graduate, so you won't have to waste time looking for a job elsewhere.
An internship can teach you practical skills beyond what you could learn in the classroom. Even though professors aim to keep you up-to-date on what's happening in the field, you may find that the only way to get hands-on experience and understanding is by becoming part of the industry. If you're unsure of exactly what you want to do, you could try out a couple of different internships and see if the job is what you're looking for.
Whether your internship is paid or unpaid depends on the company you intern for. An unpaid internship is worth it, as you will still gain valuable experience. Have questions? Ask a career placement services advisor to help you find something that fits your career goals.