The UN Commission of Experts (established pursuant to Security Council Resolution 780) held that the practices associated with ethnic cleansing "constitute crimes against humanity and can be assimilated to specific war crimes. Furthermore ... such acts could also fall within the meaning of the Genocide Convention." The UN General Assembly condemned "ethnic cleansing" and racial hatred in a 1992 resolution.
The purpose of ethnic cleansing is to remove competitors. The party implementing this policy sees a risk (or a useful scapegoat) in a particular ethnic group, and uses propaganda about that group to stir up FUD (fear, uncertainty and doubt) in the general population. The targeted ethnic group is marginalized and demonized. It can also be conveniently blamed for the economic, moral and political woes of that region.
Physically removing the targeted ethnic community provides a very clear, visual reminder of the power of the current government. It also provides a safety-valve for violence stirred up by the FUD. The government in power benefits significantly from seizing the assets of the dispossessed ethnic group.
The reason given for ethnic cleansing is usually that the targeted community is potentially or actually hostile to the "approved" population.