Making sense of the traveller

From what I've read, the purpose of the traveller is to keep the main sail centered when close hauled. Okay, but:

1: The traveller on the C14 is not adjustable (unlike in something like a Laser), so what does it do then?
2: The traveller is connected to the main sheet, so even if it were adjustable, letting it out would be the same as letting out the main sail, no? Why is the main sheet connected directly to the traveller?

Also, how long should the traveller line be? The previous owner of...

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