In The Dalek Invasion of Earth, why did the Daleks want to turn Earth into a giant spaceship?

Question

I'm specifically referring to the First Doctor serial The Dalek Invasion of Earth, not any of their many subsequent attempts to invade Earth.

BLACK DALEK: What remains to be done?

NORMAL DALEK: Only to put into position the penetration explosive. The charge is in the fissure capsule now. When positioned, the charge will strike the fissure in the Earth’s crust here.

BLACK DALEK: The fissure will expand, the molten core be released.

NORMAL DALEK: We will then control the flow until all the gravitational and magnetic forces in the Earth’s core are eliminated.

BLACK DALEK: Excellent. I will now announce to the Dalek Earth force, completion of Project Degravitate in two hours time.

...

BLACK DALEK: This is the Supreme Controller. Our mission to Earth is nearly completed. We were sent here to remove the core of this planet.

BLACK DALEK: Once the core is removed, we can replace it with a power system that will enable us to pilot the planet anywhere in the universe.

BLACK DALEK: All that remains is to put into position the penetration explosive. Daleks controlling device will now report.

Unfortunately, I don't recall anyone mentioning why the Daleks would want to move the Earth anywhere else in the cosmos.

It doesn't seem like a very efficient way to construct a super-huge spaceship. It's not much use as a weapon. If they just want to do evil things to humanity there are far simpler options. And, there's no indication that they're running out of living space back on Skaro. So, any ideas?

Answer 1

According to the Audio serials "Lucie Miller" and "To the Death", the Daleks wanted to turn the Earth into a gigantic plague ship with which to infect the universe. Having failed the first time, they repeated their plan a few decades later:

DALEK TIME CONTROLLER: All Dalek forces will return to my orbital fleet. The time warp engine capsule will then be activated remotely. It will pilot the Earth to the exact point in Space-Time where the deadly viruses have amassed.

THE DOCTOR: So the Earth is to become a plague planet, teeming with multiplying viruses. You'll wipe out all life on the planet.

DALEK TIME CONTROLLER: All life on all planets.

THE DOCTOR: Has it ever occurred to you that your trip through eternity might have evaporated your sanity, if you ever had any.

DALEK TIME CONTROLLER: The plague planet Earth will be piloted first to the home planets of all races which threaten Dalek supremacy. Every world that comes under its gravitational influence will be fatally infected, all populations wiped out.

Answer 2

According to the New Adventures "The God Engine", one group of the Ice Warriors offered the Daleks the use of the "God Engine" in return for the control of Mars for themselves. It required a planetary mass to brace it against the subspace fields it generated that was only operational in the absence of a bipolar magnetic field, and this was why the Daleks intended to remove the Earth's magnetic core. The story is interesting if only because it made a fatal flaw in sci fi, it made the Daleks technically very impressive, contrast with them on Earth when (for example) deep in the mine, a Dalek raise's it eye stalk as an antenna to improve reception!