When you are close to a building that has an interior, the building appears see through on your minimap, allowing you to see the 3D interior map.
I noticed that in GTA 6 when you are close to a building that has an interior, the building appears see through on your minimap, and you can see a 3D map of the building interior. so there is a practical reason for the 3D buildings on the map. Additionally, any buildings that have an interior of some kind, appear blue~ish on the map, instead of light grey.