"Because you can't learn experience from a book." Formal education gives you a grasp of the norm, but on-the-job training gives you what lies beyond the norm.
Well in the computing field the norm and the what lies beyond are both learned on the job. What about people who gave up lucrative careers to become writers. Their norm was never gotten through formal education?
It's religious. Our "academic" system began in ancient Greece. Akademos was a grove near Athens, sacred to the goddess of wisdom. Membership was by invitation only. Members claimed ownership of all knowledge and discoveries by outsiders were rejected. The most famous member was Aristotle, and 2,300 years later his name is still a synonym for "arrogant jerk". The only change in that time is that classes used to be conducted while walking outside, but now are conducted while seated inside.
1) It gives you the background knowledge for the subject in which you wish to work, if directly linked.
2) It proves your academic ability even if the degree course is not necessarily related to the career.
On-the-job training won't help you much if the career demands very deep background knowledge. I worked for 20 years as a lab assistant, and whilst I learnt the work itself sufficiently thoroughly my poor academic background meant I could never have advanced any further, where I would have needed to analyse the results I gained, and help design the devices I tested.
On the other hand a friend of mine completed an engineering apprenticeship, which included college work but was primarily practical training at work, then went to university, gained an appropriate degree and now has a worthwhile, very highly technical hence highly-paid career thanks to his own choice to combine such a depth of training.
Benedict Arnold 