there is much reasons depends on humans minds
increase your information about that subject
to get a better job
in some socialites you will increase your reputation or karma, respect from others
That said, I've always wanted to do one, because I've always wanted to be an academic. Why do you need one to be an academic? Well, a PhD is a research degree. You can't do research unless you know how to do research. This, loosely speaking, is what a PhD does.
(Also, I like maths...no job outside academia can quench this thirst! And doing a PhD is easier than getting a real job - I don't actually go into my office until about 1030-1100 hrs...)