Interesting. I didn't know that. Things have changed I guess. When I first started working, there were engineers without college degrees as well.
Honestly, part of it is a way to make money. And it's a way to employ people whose knowledge isn't otherwise all that useful (which is not as derogatory a comment as it probably appears to be).
But, there are some reasons - even if it's gotten a little extreme (my son is trying to push through nursing school right now). In my engineering program the professors were pretty honest that the entry-level courses were intended to test your mettle more than teach anything. They weren't going to be bothered with people who couldn't prove they could make it.
So, it fosters growth and maturity. It teaches a work ethic and learning habits that too many people don't get from primary and high school. If you use the time wisely (taking classes that actually apply to medicine) it introduces you to the paradigm of your future profession. As much as people seem to think the scientific method is some grand, all-encompassing, answers-to-all-problems entity, each profession built on the scientific foundation has its own paradigm for solving problems.
My son makes frequent comments that I need to stop approaching biology like an engineer.