This common interview question can feel like a smack in the face. It can catch you off guard, put you on the spot and make you feel exceedingly uncomfortable. Think about why the interviewer is asking this question. Essentially, he is saying: "Why should we choose you over other candidates?" Or: "What do you bring to the table that other applicants do not?"
These questions are not easy questions to answer. After all, you probably don't know who the competing candidates are or what their qualifications are. You have to make your best guess based on current and past experiences and what you know of colleagues in your field.