Because it addresses the fundamental questions of human existence.
Because it is intrinsically fascinating.
Because it instils techniques of criticism and argument at a level unknown in other subjects.
Because all other subjects presuppose answers to philosophical questions, and it a good thing to know what those answers are.
All of these are true, but they may not convince everyone. In fact, anyone thinking seriously about studying philosophy will sooner or later get asked questions like ‘how are you going to get a job with that?’.
Well, it turns out that philosophy students do get jobs, and in fact get jobs at rates that compare extremely well to graduates in other subjects, including vocational subjects. This article in the Guardian will tell you more, as will the resources pages of the Australasian Association of Philosophy, the American Philosophical Association, and the British Association of Philosophy.