Ground Plane on Top and Bottom layers in 4-layer PCBs
Yes. It reduces waste since less copper is etched so less waste chemicals are produced and is therefore more environmentally friendly. It's also faster to etch.

In the old days when PCB quality was not as high, boards would warp or twist if the copper was imbalanced on both sides of the PCB. Copper fills prevented this from happening.

Also it makes the PCB look cheap when so much copper is etched away because fiberglass is ugly.

It also helps heatsinking. Don't leave the fills floating. Via stitch them (just one via is good for low frequencies) to the ground plane or else they can reflect and cause RFI issues.

If you have internal signal layers, then the copper fills on the outside serve as shielding as well, as well as serving as a second ground plane on both sides of the internal trace.