I believe there are three common reasons are to prepare for career, increase their knowledge and get new experience.
Prepare for career is very important for young people nowadays. In a competitive society, if they have not a good educational background they will not get a good opportunity in their career or even cannot find a job, so college and university can provide them new skills to meet the requirements of career.
Also, students attend to university or college can increase their knowledge in subjects they interested, for many people, this is maybe the primary reason to go to college. There are many excellent professors and experts can give us a plenty of knowledge which including many areas. For many people this maybe their last chance for a long time to get knowledge that don't relate to their career.
Besides looking for knowledge, some people want to go to college or university because they decide to live a new life. University can support young people an environment where they can intercommunicate about many topics such as literature, goal of life, art and emotions which are hard to talked about their universities lives, so it is a special experience for young people to live and study in college or university.
All in word, I prefer increase knowledge and get a special experience to prepare for career in university because getting good opportunities in career is partly depend on your knowledge and experience on diverse areas.