Because multiple devices (nodes) are connected, the ground potential at different nodes could be different, which can cause ground currents to flow through “the path of least resistance” via either the common earth ground or the ground wire.
Both voltage and current surges can cause serious damage to valuable, mission-critical equipment such as computers, PLCs, and industrial manufacturing and data processing equipment; these surges can even endanger human lives. Ground loop problems also exist on unbalanced signals that rely on the ground pin being zero volts, such as RS-232 and USB.