We all know it's coming, yet we may not be fully prepared for it. The moment in the interview when the human resources representative/committee/interviewer asks "Why do you want this job?". Hopefully you have prepared and have a solid answer, if not you may want to read this from the Undercover Recruiter and how to answer the question.
In the article there a four steps to help you prepare your answer, but it seems to me the two keys are use the question as your chance to make your sales pitch (i.e. highlight why you are the perfect candidate) and second you want to know what they want in a candidate.
What this article fails to tell you is that in order to know what they want in a candidate you will undoubtedly need to do some research about the company, position, and possible the industry in which you hope to work. I have written pieces in the past about this type of research, the most useful may be this on Company research - business news.
Some additional things to remember are the power of LinkedIn and other social networks. Before the interview you may want to search by job title to see the career path of others in the career you are pursuing. You also will want to use Glassdoor.com or Career One Stop to look up salary data and interview question styles.
Use all the tools that are available to you, go into the interview prepared, relaxed and confident and hopefully everything will work out.