AC Electric Neutral Current "Flow"

Hi everyone

Can someone point me in the right direction here please. Happy to do the digging, but so far I can't find anything that helps!

In an AC alternating-current electrical circuit, the Neutral wire is ultimately connected to "mother earth" / "big stake the ground" - sometimes at the household (e.g. UK), sometimes at a distribution point (e.g. europe). I've used "mother earth" as this is NOT a question about the "earth" wire you find in a plug...!

So... Into my simplistic circuit (let's say from the left) comes my Live 240v / 120v wire, goes into my appliance (toaster / light bulb), out from the right of the appliance comes a Neutral wire that somewhere way over to the right ends up buried 6 feet in the ground.

So far so good. The bit that is doing my noggin in, is the neutral line has a zero voltage. I can not get my head around the How & the Why?
- How can I have 240v going into a light bulb and 0v coming out?
- Why does the neutral need to be grounded if it's Zero volts (what part DOES it play in the circuit?)

I know I've picked up a wrong concept in here somewhere... any help in finding it would be most appreciated!

Thanks
Matt