Of course, mass migration has many uses or nobody would promote it. For instance, it exorcises, somewhat, “the specter of uselessness”* that haunts so many people in the liquid world.
The arrival of migrants requires an army of bureaucrats, state officials, social workers, etc to process their paperwork, hand out new paperwork, file the paperwork, and come up with new kinds of crucial paperwork. In this way, you can occupy them for years, making it look like what they are doing is extremely necessary.
This is a way of pushing many locals out of the danger of slipping into the welfare class. For this purpose, a cushion needs to be created in the form of a much more dependent and helpless welfare class.
The longer the migrants stay unintegrated, the longer can this game be played. A self-reliant, independent migrant is useless in the game of providing the superfluous locals with the sense that they are not all that superfluous if there are people who are even more easily discardable than they are.
* The term was created by Richard Sennett.