Jobpostings Magazine asked Christine O'Neill, Human Resources Manager at CN, about one of her favourite questions that comes up during interviews:

If you're hired, what will be your impact?
This is a seemingly harmless question we ask at the onset of the interview, but it provides us with a wealth of information on the candidate when answered strategically.  
First, this is an opportunity for the candidate to tell us about the strengths they've developed that could be an asset to our organization.  You should focus on one or two strengths, and provide examples where you've demonstrated these specific strengths in the past. Simply listing your strengths without facts to back them up makes the answer fairly shallow.
Second, the savvy candidate, who has gathered information on CN's goals and culture, will also be in a position to tell us how they would be a good fit within our environment. CN's website (www.cn.ca) offers the public a lot of information on its mission, its challenges, and its opportunities. A candidate who has taken the time to read our information will be well equipped to explain how they can contribute to CN's success.
The answer should also remain specific and realistic (no, we don't expect the candidate to revolutionize the industry in their first week on the job). Not being able to explain which aspect of our business you'll be most beneficial to, or advising us that you would change everything about the business leaves the interviewers with the sense that you haven't done your homework. Or worse, it shows us that you wouldn't have an impact (which is, after all, the question you've been asked to answer here).
Finally, the successful candidate will be the one who uses this answer as a thread throughout the interview. If you've been candid about your core strengths in this question, then this theme will invariably resurface throughout the answers you provide as the interview progresses. If you've simply presented a generic response, then the interviewer will automatically notice your answers in the remainder of the interview don't match up to the candidate you presented yourself as from that first question.
As I said, this is a seemingly harmless question, but it requires some preparation and candidness on your part.  Know yourself and the organization where you are interviewing, and you should be successful in dealing with this type of interview question.