Going to graduate school is fun! Below is my proof:

Learning from your advisor, who is typically brilliant.
Working with other smart people.
Be forced to master the most advanced material by professors and TAs.

And you not only do these for free, you actually get paid for doing all these! Can you find a better deal?

To have a successful academic career: 1. know how to do good research, of course 2. know how to write good grant proposals! I claim this is even more important than pt 1 in some sense. p.s. actually I think those two are highly positively correlated.

following was shamelessly copied from http://www.cs.cmu.edu/~scsfacts/vonahn-simon.html.