Dipak wrote it nice from CMP perspective.
However, copper-process is around for last 5-6 years or so. Achieving uniform Metal density requirements are there since Aluminum metallization days. CMP was not used those days to achieve planarity. Even conformal dielectrics have been used.
The core reason - Interconnect parasitics need to be predictable within an acceptable error limit.
The more the planarity is achieved on the top of an ILD - more closer will be extracted RC prediction, with real measurements. Ancient CVD [Chemical Vapor Deposition] days - closely placed dummy metal islands help maintaining a uniform overall planarity of dielectric layer, which in-turn helps next metallization to be nearly planer - and the next ILD and so on..Density is a point function - it was more important to measure and achieve density within smaller window-size in lower metal-layers, than upper ones.
The dummy metal-fill patterns are not useless too - they can be utilized to serve as de-caps, serve as shields, thermal-distribution.