The motive of this question is to understand if you are really motivated to be a surveyor or is it just a second choice of career for you? The answer to this question has a lot of importance in this case as being a surveyor requires a lot of hard work and patience involving many hours of work outside the office in scorching heat or cold weather and a lot of paperwork as well. Someone who is not really motivated to do this work might wither off quickly and may end up leaving the job very soon. In which case the hiring manager may not be really interested in hiring and training you for the job.
Talk to the hiring manager a bit about the path that brought you here, what do you like and enjoy the most about this job and any other positive points of this job that you may want to mention. You can also go through the job description and pick some interesting bits just to make your answer more appropriate but make sure it doesn't sound like something you just said in order to impress the hiring manager.