it completes the circuit. Lets say you could unplug everything in your house except one 60 watt lightbulb. When you turn on the light 60 watts of power flows from the X1 bushing through the bulb and then back out to the transformer to the X2 bushing using your neutral. Its a circle or complete circuit. This happens with every 120 volt circuit in your house. Lets say you have a 240 volt hot water heater in your house in this case your using the X1 and X3 bushings with a resistive load in between, in this case current flows through only the hotlegs.
Now things get real strange when you start to loose a neutral connection. If current has difficulty using the neutral to get back to X2 in a 120 volt circuit its gonna look for other ways to get back to the transformer. That means its gonna start using the other hotleg through a resistive load to get back to the transformer.
When you get to your first bad neutral call its always the same complaint. The customer will tell you half the house is real dim and the other half is real bright. when you get the fluke out and test the hotlegs under load you might find 80 volts on one leg and 160 volts on the other. (240 volts phase to phase).
loosing a neutral gets real expensive for a customer. In a blink of an eye you can lose your TV's , microwaves, fridge, stove any any thing else plugged in and in use at the time. a straight 240 applience like a heating element doesent know the difference except suddenly its got way more current flowing through it which may burn out the element.
hope this simple explanation help you out.