You are not clear of whether it is in antennas or pcb. If you see it in antennas and radiations , it acts as a mirror in monopole antennas. It replicates the pattern provided by monopole antenna making it to look like a pattern out of a dipole antenna. In the case of pcb it acts as a return (or closed ) path for the current. making it a closed circuit to make the circuit work properly. In either cases, it need not be that the ground reference plane is zero all the time. It can also be any value other than zero also. But when you consider other value, you should keep in mind that you have got a non zero reference value in your calculations and you should perform the calculations accordingly.
