Good grounds & good wires are important because poor grounds & poor wires cause
voltage drop. The consequence of this
voltage drop is that the systems in that circuit can malfunction. How do bad grounds/wires cause voltage drop? Well, the answer is Ohms Law.
Ohm's Law states that V=IR. This means that the voltage drop (V) is proportional to the current through the circuit (I) and the resistance (R) of the
resistor (or bad ground in our case). An easy example is attempting to draw 10 amps through a bad ground with a resistance of 1 Ohm. So V=10(1) thus making the voltage drop a whopping 10V. This leaves only 2 volts available (in a standard automotive 12V system) to power the circuit and means that all that extra power must be dissipated as heat at the resistor (bad ground). Generally this heat makes the condition even worse, causing more voltage drop and more heat. In the most severe cases the ground will function under light loads yet fail under high current draws causing what appears to be an
intermittent failure. Many people have seen this behavior caused by a loose (high resistance) battery terminal.
Bad grounds/poor wires can cause poor performance in almost any circuit. Voltage drop can manifest itself as dim headlights, one circuit malfunctioning when another is switched on (a common
symptom is that the heater fan slows and dash lights dim when the
brake pedal is pressed) and all kinds of other random weird issues.