Hi, what is the purpose of the timing belt, and why does it need changing?

Your kidding right.....................but here goes:

Timing Belts are made of a rubber based belt with high strength strands of Fibreglass running around the complete length of the belt. They have teeth moulded into them, & their sole function is to drive the necessary components, such as camshafts. They have to maintain correct timing of the components, if they go out of time the Inlet & Exhaust valves in the cylinder head can come into contact with the pistons coming up, causing an expensive repair bill.

I've just given you a brief explanation but thats it basically.
Any more detail and i guess you wont sleep at night