Why is University so wanted? Can someone tell me why? I've never played it myself so I'm interested.

It's part of my Sims circle of life.
I really don't start generation play until I get university.
University is where my young sims are shaped as far a personality and careers are concerned.

In the past expansions, University brought more careers and gave sims additional traits and want slots.
They usually meet their future spouses there too. University also adds a change of regular game play after you have played the parents through their careers and the kids are grown up.