actually it is measured so that we could guess the incertanity and poor performance of the object we want to make by this metal. usually it is necessary in mills and heavy machines
Thats a good question i faced same question in interview but in different way..
if we have more parallel vias between two metals.. then the metal density will not be uniform...
Actually there must be some minium density in a metal.. according to drc rule.. if u have more parallel vias.. then in that place the metal density gets low as vias present more...
There must be some metal density for electrons to propagate...
Another thing that brings about the metal density DRC rule is the fact that, mechanically, each layer of metal has to have a certain metal density in order for the chip to not have reliability issues due to mechanical damage resulting from heating/cooling and different therman coefficients of the various layers.