(1) Description of the pattern
One of the best ways to exploit and mistreat a population is to distance oneself from them as much as possible. We can justify our actions against others by portraying our enemies as sub-human monsters that deserve less that ethical treatment. By creating rigid us/them boundaries we can effectively establish that "we" are in the right, and "they" are dead wrong.
(2) Why the pattern is good (i.e. bad)
Dehumanization is an effective method of controlling the populace. By continually treating and portraying subsets of people who aren't in line with the goals of the state as less than human, that representation will eventually permeate the minds of the rest of the population. As the rest of the population adopts the dehumanizing attitude, they will naturally treat the target peoples with malice. This will serve to reinforce the aims of the state and will create an institutionalized oppression.