Copper weight is critical in controlling the circuitry widths. Thinner copper reduces the total amount of etching, while the process does not dramatically effect the base line trace width, the top is affected.
The even distribution of copper can also assist with thermal conductivity of a particular design, as parts become smaller and more demanding, the watt density of the power being generated on a board increases. Some of the failures of critical devices could be attributed to the lack of thermal relief. Without providing a sufficient path to extract the heat away from such devices, in turn, reduce the lifespan of a particular chip set.
The even copper distribution throughout the construction of the board is not limited to only function. By distributing copper evenly from inner layers to outer layers you can not only assist manufacturing with etching processes but also with plating. If left unbalanced you could end up with a board that is going to be over / under etched, over plated and potentially even warped / bowed.
One should consider the following: Power handling, how much is required, impedance control, as well as the width of traces and spaces in between, it typically would require
thinner copper thus less heat dissipation.
Common practices among manufactures are to add copper thieving throughout the board if the design engineer has overlooked this aspect of the layout. Caution must be taken to ensure signal interference is created when thieving is added. It is recommended that you seek the approval of the layout engineer prior to commencing with the fabrication of the build.