This is your interviewer's way of knowing about two things. Firstly, he or she wants to know how dedicated you are towards your career; and also how realistic. There is a huge difference between being ambitious and over ambitious, so keep your answers practical and attainable, but they should in no way project you as a hare. Secondly, your interviewer wants to know how the company will benefit from you achieving your goals than what you'll get from it, but it is one and the same thing, to a large extent. It is a bad idea to tell your potential employer that you plan to go for his or her job, but it is okay to mention that you'd like to earn a senior or management position.