Internships gives you an opportunity to implement your theoretical knowledge practically..It gives you an exposure to real technical world..Many times it may be a case that what we study may not be practically used.So as to keep ourself updated with real engineering we tend to give inportance to internships.
Just to get some experience of professional life beforehand. Also sometimes u get paid for it. And the experience u get can be showed in your resume for jobs , after u finish your degree. If you choose the company related to ur branch , then you also get the opportunity to use ur knowledge in real life situations.