Different people have different answers to this question. I believe that the three most common reasons are to prepare for a career, to have new experiences, and to increase their knowledge of themselves and the world around them.
Career preparation is becoming more and more important to young people. For may, this is the primary reason to go to college. They know that the job market is competitive. At college, they can learn new skill for careers with a lot of opportunities. This means careers, such as information technology, that are expected to need a large workforce in the coming years.
Also, students go to colleges and universities to have new experiences. This often means having the opportunity meet people different from those in their hometowns. For most students, going to college is the first time they've been away from home by themselves. In addition, this is the first time they've bad to make decision on their own. Making these decisions increases their knowledge of themselves.
Besides looking for self-knowledge, people also attend a university or college to expand their knowledge in subjects they find interesting. For many, this will be their last chance for a long time ti learn about something doesn't relate to their career.
I would recommend that people not to be so focused on a career. They should go to college to have new experiences and learn about themselves and the world they live in.