Survey is used to identify the exact location and the boundary of the parcel for your property. Mortgage company requires the survey to make sure that the improvement on the property such as houses are really located inside the parcel within the boundary.
It's also good for you to understand how your property is adjacent to your neighbor's. In Urban area, survey is normally not a big deal since the property is all fenced already unless there is dispute about the boundaries. In Rural area or vacant land, it's hard to tell the exact boundaries. Survey in this case will help even more.
