basically people do that tom make sure that they do not lose slaves quickly, although it cannot be done instantly as u cannot enter VM until at least 24 hours after u`ve attacked sum1.
basically its so medium power empires keep slaves of their own power by reaching a high PR and keeping it over a long duration of time. basically ensuring a high power that they would not normally be able to maintain and henceforth keep the slave to get lots of money! =P