Hi,
* as a plane it shields electric fields.
So that the noise of a trace can not go "through" the plane to the opposite side.
* magnetic field / impedance
It´s not the retunr path alone, it´s the combination with the signal path.
A signal in a trace causes a current and thus a magnetic field. Causing an inductance.
With a GND plane, the "return" current flows in the GND plane in very close proximity to the signal.
The current direction in the GND plane is opposite to the signal current. And so are the magnetic fields, thus thy cancel out each other. No magnetic field, no inductance, no transmitting antenna for EMI
Klaus
added:
This is an often discussed topic, thus there are many hits on an internet search. Besides a lot of good documentation you will also find videos with explanation and according simulations.