This is essentially the same question as 'Why did you apply for this job?' & 'Why do you want to work here?'. Reply to it by showing that you have researched the employer and have logically thought out reasons why you think you are a good match for their requirements. Turn the answer into a compliment to the company you are interviewing for.

☛ There are only a limited number of opportunities for advancement with my current employer. Which is why I'm keen on working for a larger corporation like yours where I believe there are more possibilities to show what I'm capable of.

☛ I'd like to work for a company where I feel I can make a real difference.

☛ A common way to reply to this is to research the company before hand, find something unique about them and then give that as your answer. For instance they may have opened up a new store or launched a new product.

☛ Give examples of positive things you have heard about them and say you want to be associated with a reputable brand.

☛ You believe their work environment is more fun, energetic and rewarding.

☛ My main reason is financial, my complimentary reasons are the opportunity to work for a market leader and also future promotion.

☛ After researching your company I have discovered that it is a industry leader and has a excellent reputation. I was also greatly impressed by your companies mission statement, values and culture. I feel strongly that yours is a organization that I would like to work for and be associated with.

☛ I feel that your vacancy ideally fits my work experience, skills and qualifications. Therefore I will be able to make a significant contribution to your business as well as fulfill my potential.

☛ I believe your company will help me to develop my career in the direction I want it to go.