It's a map. Buildings are landmarks. Remember it's not just for routing your car journeys, it's meant to have everything on it. You look at the map, you see a square building. Is it a warehouse, a skyscraper, a radio antenna? Will you be able to see it from a mile away, and if so what will it look like? That's very relevant info. It might seem odd to be collecting data on people's homes, but it can't even guess that it might be somebody's home without asking you such questions. And the info it asks for is public anyway, it's not like you can't find it all out on Street View.
