He is trying to remove sludge deposits. Modern oil filters should do a really good job of removing all the solid contaminants that are large enough to cause damage or wear in your engine. What is left are smaller contaminants that are easily enveloped by the oil causing very little if any damage or wear, but cause your oil to turn dark. If you are concerned about your oil, you should do an analysis, as looking your oil doesn't tell you much. You might discern from looking at it that the oil is really new or really old, not much else. (or a major problem such as coolant in the oil)
