1.EMC.For large areas of power supply laying copper, will play a shielding role, such as PGND play a protective role.
2.PCB process requirements. Generally, in order to ensure the plating effect or the lamination is not deformed, the PCB board with less wiring is coated with copper.
3.Signal integrity requirements.Give high frequency digital signal a complete backflow path, and reduce DC network wiring. Of course, there are also heat dissipation, special device installation require to lay copper and other reasons.