While most of the replies are spot on, here is a practical example.
Say you have an ADC chip that uses ground as a reference. You run the ground signal along a trace which joins a trace that has a high-current device on it. The combined trace then continues to the power supply ground.
Since the trace has resistance, when the high current device is operating it will cause a voltage drop along the combined trace which will lift the ground to the ADC by I*R, throwing off the ADC results.