'What about the prisoners, then?' she said. 'Why keep them here?'
The Doctor shrugged. 'Biological matter the can use to construct new, mutant Daleks, or fodder to test their experimental weapons. That's all, I'm afraid.'

[...]

'And this is what they'll be doing on all those other planets in the Spiral?'
'Probably,' said the Doctor, 'or they'll have sunk mines and enslaved people to dig, harvesting minerals and precious metals for their war effort. They don't have a great deal of imagination, unless it's to do with killing people.'