dummy metal filling
There is alot of usage in dummifications,
We can dummified the Metals, the VIAS(contacts between metals), and even the base layers such as diffusions, polys and nwells.... WHY? All above that mentioned is the answer, I will add on a few more, as during fabrication, materials are doped layers by layers, ....
Exclude the substrate/bulk... the other areas that didnt doped by materials that use for current flows is what we called the Field Oxide (fox) or ILD. This layer are soft, we need to dummified some metals enable to strengthen them, this is because when planarization phase in fabrication will cause the soft area tend to have some "valleys".
For better understanding, imagine dummy Metals as BRICKS, and the soft areas as SIMEN.