It's a basic principle of anarchism, so I suggest reading some anarchist literature. But the gist is that such systems of oppression serve to maintain class divisions and reinforce the hierarchies of power that allow the economic exploitation. The exploitation, in turn, obviously allows the oppression to continue by allowing those in power to hold the basic necessities of the exploited ransom in order to coerce their labor from them and control the surplus. Those who are the worst off are the most exploitable, and provide the most profit. Overt differences between people also become convenient excuses for using fear to turn some segments of the working class against others.

Richard Wolff stated it well recently. If periods of greater economic strife (recessions) had a good chance of hitting any industry, or demographic, or class of people in general, then everyone is going to get angry eventually, and you'll eventually have a revolt on your hands. If, on the other hand, you can purposefully fix things so one segment gets hit hardest most or all of the time, and you can convince the rest that they deserve it in some way, then you can simply create a class of dispossessed and poor, and let the working class maintain the divide with little additional effort on your end. Divide and conquer. In the U.S. this target group is basically black people (and to a somewhat lesser degree other minority and economically unprivileged demographics).