This merging cleanup-functionality can also merge polygons that represent vertical surfaces (e.g. walls) that are above/under each another. In particular, after importing 3D-meshes representing buildings, you will have, for each building, a bunch of triangles; this functionality will help you to merge the wall-triangles. When merging such 3D-mesh buildings, make sure you select the option to convert the XYZ-vertical polygons to XYZH-vertical ones. Also, it usually helps (esp. for SwissBuildings3D) to use the option to merge only elements having some vertex with H≥~0.30. See SwissBuildings3D.