“Why should I hire you?”  This question has been ranked by Forbes.com as the most difficult interview question to answer.  This question is testing whether or not you are capable of doing the job, ready and committed to do the job, and if you will be a good fit for the job.  While this question can be tricky to answer, the answer you give is of utmost importance to the hiring authority.  You do not want to be vague in your response.
Bob McIntosh, of recruitingblogs.com, has detailed how to nail your response to this tricky question in the article, “How to Answer One of the Most Important Interview Questions.” This is a must read if you are in the interview process.