This is an important question and it's one that in some form is asked at just about every interview. You need to have an appropriate answer ready, preferably you'll have a story that will demonstrate what you are stating. Answering it (like I had many years ago) with “well, I need a paycheck will probably not be successful and so you've wasted your time. Personally, I don't like wasting my time. Also, answers like “I need a job” won't work very well either. They know both of these so you're just stating the obvious, that's the same reasons your interviewer originally applied there. Even if it is rather hokey state something along the lines of “so I can't improve my skill-set further”. If you use this answer be prepared to have a story ready where you did just that at some point in the past.