A typical answer would be: I am interested in your PhD program because I want to do research in my area. My career goal is to become a professor at a university or research institute. Indeed, doctoral degrees are by default the training programs for academics.
Why do you want to pursue PhD answer Quora?
You feel a vocation to research a particular subject and wish to create knowledge. You wish to prepare yourself for a career in academia. You enjoy academia and wish to delay having to get a job in industry. You wish to advance in a career (other than academia) in which a PhD is a highly sought credential.