1. EMC can shield large areas of ground or power supply copper, and some special areas, such as PGND, play a protective role.
2. PCB process requirements. Generally, in order to ensure the electroplating effect or the lamination does not deformPCB boardLayer copper.
3. Signal integrity requirements: provide a complete return path for high-frequency digital signals, and reduce the wiring of DC network. Of course, there are also reasons such as heat dissipation, copper coating required for installation of special devices, etc.
Advantages of copper laying on pcb 
One of the advantages of copper laying is to reduce the ground wire impedance (a large part of the so-called anti-interference is also caused by the reduction of the ground wire impedance). There are a lot of spike currents in the digital circuit, so it is more necessary to reduce the ground wire impedance. It is generally believed that the circuit composed of all digital devices should be laid on the ground in a large area, while for analog circuits, On the contrary, the ground loop formed by copper laying will cause electromagnetic coupling interference which is not worth the loss (except for high-frequency circuit). Therefore, not all circuits need to be paved with copper (BTW: the laying performance of mesh copper is better than that of the whole block).