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

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?