The Earth is a well known hub of anti-Dalek activity. Note that their [primary targets][1] seem to be facilities and locations that have shown an ability to defend the Earth against alien assaults in previous serials:

 - **Unit HQ**
 - **The Valiant**
 - **Torchwood**
 - **New York** (HQ of the United Nations)
 - **London** (the site of the Torchwood anti-Sycorax weapon seen in "[The Christmas Invasion][2]")


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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!*


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They also need to "harvest" a considerable number of humans (including Jackie, Mickey and Rose) to test their destructo-ray on.


  [1]: http://web.archive.org/web/20120330161328/http://leethomson.myzen.co.uk/Doctor_Who/Doctor_Who_4x12_-_The_Stolen_Earth.pdf
  [2]: http://tardis.wikia.com/wiki/The_Christmas_Invasion