The Earth is a well known hub of anti-dalekDalek activity. Note that their primary targets are allprimary targets seem to be facilities and locations that have shown an ability to defend the Earth against alien assaults in previous serials:
- Unit HQUnit HQ
- The ValiantThe Valiant
- Torchwood
- New York (HQ of the United Nations)
- London (the site of the Torchwood anti-Sycorax weapon seen in "The Christmas Invasion")
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.