Can anyone tell me what the actual effect of high current and high voltage are in actual circuits?

As for heaters, a heater with high power can make the whole room feel warm, but a heater with low power cannot. High power means small resistance and high current. Under the same circumstances, it can do more work and send out more energy. Big energy.

If the voltage is too low, the load cannot be carried, but if the voltage is too high, the load may be burned. This is the same as water pipes. If the water pressure is too low, the water will flow slowly. If the water pressure is too high, the pipe may burst.