Sociological explanation is needed because there are some influences on human behavior that cannot be explained solely through other types of explanation.
Let us take, for example, the fact that there is more unemployment among non-whites than among whites in the United States. We no longer accept the idea that biology makes one "race" superior to another. We could use psychological explanations and say that the unemployed people have value systems or ways of thinking that make them less likely to be employable. But even if that is true, why is unemployment more prevalent in one group of people? Clearly, there is something about that group and the position it holds in our society that make it more prone to unemployment. This is a sociological explanation and no discussion of race and unemployment can be complete without it.