Power interfacing question

[Skylinegodzilla]

If I have a breadboard power supply do I need to use the power pins on the pi?

So for example say I had a sonic sencer on my breadboard that was powered by a breadboard power supply. If I have the eco pin connected to a gpio pin on the pi (using a voltage divider of coarse) do I have to complete the circuit by using the pi power pins or will the circuit still work if there is nothing connected to the power pins of the pi?

The reason for my confusion is because with my understanding of electronics your voltage is the amount of power between two points but with a lay out like this there is 3 points 2 from the breadboard power supply and one to the pi gpio pin.

[rpdom]

At the very minimum you need to connect the Pi 0V/GND to the sensor 0V/GND to complete the circuit.