In the Season 4 finale, Davros and Daleks teleported Earth into Medusa Cascade. They were going to use Earth as a part of transmitter to demolish the reality itself.
What was the point of invading the Earth?
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Sign up to join this communityThe Earth is a well known hub of anti-Dalek activity. Note that their primary targets seem to be facilities and locations that have shown an ability to defend the Earth against alien assaults in previous serials:
They expend a considerable amount of time looking for the Doctor
DAVROS : I mean, is there news of him?
SUPREME DALEK : Negative! No reports of Time Lord. We are beyond the Doctor's reach!
They also need to "harvest" a considerable number of humans (including Jackie, Mickey and Rose) to test their destructo-ray on.