Understanding free movement and migration in the European Union
Common rules link emigration and immigration
States can — and often do — design their immigration policies in isolation of their emigration policies. It is not uncommon, for example, for countries to call for greater protection and equality of rights for their nationals working abroad while at the same time not granting these same rights and protections to foreign nationals working on their own territories.
EU freedom of movement means that all countries have to play by the same set of rules. This reduces the likelihood of states pushing for different rules for immigrants and emigrants, because if protections for immigrants are reduced in your country, this will then affect your citizens living in another EU member state. If your country is a major receiver of mobile EU citizens, but sends very few, or vice versa, this will affect your views on which sets of rules need to be tightened or loosened, and this can be a subject which creates disagreement between Member States.