Typically, most 3D maps are visualization techniques to place empty 3D building objects on-top-of traditional 2D maps. Voxelmaps is different, it creates true 3D volumetric data with an added dimension of time, not just pseudo 3D data or 2.5D data.
Using the voxel approach, the volume of each city is mapped and provided as a full, 3D virtual model with accuracy levels down to 2cms.
In addition the voxel approach allows data to be stored in multiple temporal states. This allows cities to detect change over time.