The interviewer’s job is to hire the best person for the position. Most of the candidates that make it to the interview stage are qualified for the job. The winning candidate must be more than qualified, especially in a very competitive job market.
Every hire is a risk for the company. Your interviewer will also be taking a personal career risk in recommending a particular candidate to hire. If the candidate performs well, Mr. Interviewer looks brilliant and gets a pat on the back (and maybe a bigger annual bonus).
If the candidate turns out to be a dud (doesn’t perform well, doesn’t get along with the team, leaves the job prematurely, etc.), the interviewer looks like a dummy and his professional reputation suffers.
With this question, your interviewer is asking you to sell him on you and your status as the best person for the position. Make his job easier by convincing him that:
You can do the work and deliver exceptional results
You will fit in beautifully and be a great addition to the team
You possess a combination of skills and experience that make you stand out from the crowd
Hiring you will make him look smart and make his life easier