It's done to ensure that you get the right part, and it's also done for liability and legal reasons. Lets say you mow down a pedestrian in your Legacy wagon. A witness say that they saw somebody tearing off in some kind of wagon.
You show up at the dealership the next day to replace your hood, right side fender and your headlight or whatever. It's the same reason body shops need an accident report to do any work on your car. Works for hit and runts etc.
It's not a big deal really.