What reasons make a person get a Ph.D. in a subject?
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...)