(Since this is actually a good question, you will get a good, physicaly plausible answer)
Nothing special. They had to temporarily leave the Earth's atmosphere to get a clear field of view for using the ship's x-ray laser, to send data to the center of their civilisation, which lives in the center of the Milky Way, and feeds stars to the supermassive black hole there and uses all the energy released to do whatever such a terribly powerful civilisation does.
(now a stupid question)
What is Dil's purpose, actually?