The person interviewing wants to know that you are interested in this job specifically and not just picking up any job you can get. By asking this question, they are making sure that you understand the job description and are applying because you have qualities and experience that is relevant to this position.
Points to Emphasize
A job interview is an opportunity to test compatibility between you and the company. Remember these points as you respond.
Mistakes You Should Avoid
Avoid doing those things that will make you appear disinterested or unknowledgeable about the job and company.
A great sample answer addresses specific traits and qualities of the job and the company while showcasing your experience and how it applies, like in the following example.
What interested me most about this position was the opportunity to work with first generation college students. I have always had a keen interest in education and as a first generation college student myself, I can appreciate the value of working hard to achieve educational and career goals that are emphasized in your mission statement.
The most important thing to remember when answering this question is that they want to know you want that job and not just any job. If you apply for and get a job that you want, you will be more likely to stay long-term.