This question is asked because the interviewer needs to find out what information you know about the job and company. You better know your facts before you answer this question.
Here is what you need to know:
About the Company
About the Job
Find out as much as you can about the company. Refer to their mission statement their stock history if they have one, trade publications, articles etc... Find out who the executives are and learn about their background. Learn what the company does, how they do it and for whom.
Know the job description inside out.. Find out as much as you can about it. Do research to find out the salary they are paying, who you will be working with and how the job relates to the overall company model. . See if you can even find out why the position is open.
Now that you know what the company is looking for, you can pull from your background similarities that you have between your background and this job. You can now use the company background information you have and the job description to explain why you want to work there.
Now, take 3 points from your background to explain yourself. For example: They are looking for a customer service rep. You are one at a company that is in the same business as theirs. That is two positives you have towards the job. Perhaps they want a leading sales rep and you are the top sales rep in your company where you service certain comparable companies. you can you name some of the companies they also market to, your value rises Match up what you know about yourself to the job using your experience and knowledge about what the new company is searching for.
Example: The reason I want to work for your company is first of all, it is very stable and as my background reflects I am looking for a stable company. I have a strong sales experience selling widgets and I know your products inside and out. I know I can come in to your company selling these widgets from day one. Companies like...
Remember, know the company, know the job and match your experience and you will be sailing on your way when they asked, "Why do you want to work here."