The Dalek Invasion of Earth - S2-E2
Question: Why did The Daleks want Earth? What was it that made them specifically choose Earth to invade? Also, why did they intend on turning the Earth into a massive spacecraft when they already have their own fleet?
Chosen answer: In the mockumentary "Whatever happened to Susan Foreman?" it is sugested that some Daleks may have survived the events of "The Daleks," rebuilt the race and gone back in time to take revenge on Ian and Barbra by invading 20th century Earth, but suffered a fuel shortage which left them stranded in the 22nd century. Presumably that would also leave them incapable of interstellar travel.
Josman
They wanted to remove the plants core - once removed, they can replace it with a power system that will enable them to pilot the planet anywhere in the universe. I don't know if anyone has mentioned why The Daleks would want to move the Earth anywhere else in the cosmos.
Dan23