Different people do so for different reasons. Some typical reasons are new experiences, career preparation and increased knowledge. In my opinion, it is the fast development, almost explosion, of our world that make people to consider attending college or university.
People would usually get bored when they focus on one field for a long time. For this reason, they go to college to pursue experiences in new areas. We often hear that an electrical engineer went to a university to study art or music. With the new experiences, they expand their ken. Maybe some good ideas would jump out during their daily work.
As for the young, they attend college for career preparation. Not only could they get knowledge needed for their future work, they could learn how to think and how to solve questions. The projects students attended are very like their future work.
Some people, who have been graduated from universities for years, are also planning to go back to school. Most of them attend again universities to increase their knowledge. Technology is developing so fast that people could not keep up with unless they keep increasing knowledge.
All in all, people, who attend college or university, are preparing for their better futures.