Both businesses and students benefit from internships.
For the student, an internship allows them to learn about a potential career from a 'real-world' perspective and perhaps earn some money after classes or during the summer.
For businesses, however, the potential benefits are much greater: besides getting part-time workers at an affordable cost, businesses can benefit from the enthusiasm and new ideas of a college student. And if that student has a positive experience with your company, they are more likely to end up as a full-time employee of yours down the line.
The California Chamber of Commerce recently published an article that tends to focus on the fact that internships should be paid. (It's difficult to establish all of the criteria needed to provide an unpaid internship).
And the most important thing to remember is that if an intern is paid, they are an employee - subject to workers' compensation and all of the federal and state protections from harassment, discrimination and retaliation in the workplace.