You may just really need the money for a car or rent, but the interviewer wants to hear your motivation that goes beyond monetary expenses. Maybe you want to continue your education but you don't have support from your relatives. You might have goals of becoming a hair stylist, but the training costs money. You just need a job to help you get that much closer to reaching your goal. Interviewers love to hear that you are career oriented and that you are willing to work hard to achieve your goals. You may be looking for experience that will help you get an internship or into a highly competitive school. Explain your motivation and how this job will help you get where you want to go!