Starlight has a question: why is it ponies do all that they do? She goes to Twilight who doesn't have an answer, but maybe a way to find out.
in the name of exploration, she will have to go to the end of the world and find out what the existence of her world is leading to, what the purpose of its existence is.

She does find out an answer, even if it's not the one she would have first expected.