This question needs to be carefully answered as it is your opportunity to stick out from the rest of the applicants. You should focus on skills that you have, including those not yet mentioned. Simply responding "because I'm really good" or "I really need a job" isn't going to work. You shouldn't assume the skills of other applicants or their strengths, focus on yourself. Tell the interviewer why you are a good fit for the position, what makes you a good employee, and what you can provide the company. Keep it brief while highlighting achievements.