Benefits of grounding in PCB are:
Reducing electrical noise
Reduce interference being coupled from one part of the circuit to another.
Reducing crosstalk between adjacent circuit traces.
1) Noise reduction:
When circuits switch states, large current flows from the active devices through the ground. If the power supply and ground traces have noticeable impedance, then the voltage drop across them may create noise voltage that would disturb other parts of the circuit. The large conducting area of the ground plane has much lower impedance than a circuit trace, so the current causes less disturbance.
2) Interference & Crosstalk reduction:
When two traces are placed too close to each other, an electrical signal in one can be coupled into the other. This is due to electromagnetic induction caused by the linking of magnetic field lines from one trace to other. This phenomenon is known as crosstalk. When a ground plane layer is present underneath the circuit in the PCB, it acts as a transmission line for the trace. Thus the direction of current flowing through the ground plane is opposite to the direction of current flowing through the circuit trace. This cancels most of the electromagnetic fields and consequently reduces crosstalk.
Many of the present day circuits consist of both Analog and Digital subparts. To avoid current from one subpart to affect the other subpart, ground planes are split and then connected by a thin trace. The thin trace has low enough impedance to keep the two sides very close to the same potential while keeping the ground currents of one side from coupling into the other side, completing a ground loop.