There will be a point in the mid of your interview, where your interviewer has already asked you about your strengths. Then asking this question means he/she would like an answer that is more specific to your job. This answer will help the interviewer to compare what you bring to.
You can prepare your answer by listing your strengths and connect them to the requirements mentioned in the job profile. You can say " I have already mentioned my strengths, but if you were to ask me something unique which I ring, I would say It's my attention to detail. Though it sounds wired, this strategy of mine has saved several situations.
Hope this answer will help.