You can judge whether the circuit is unblocked, whether there are disconnection, disconnection, and road failure.  
You can determine whether the working voltage of the system is normal and whether there is a high or too low voltage;  
You can judge where the circuit failure is, and the circuit voltage of the problem will be more or less abnormal, which is convenient for determining the problem.  
Realizing the significance of measuring voltage, many problems may be solved from judging voltage!