Cataclysmic variables (CVs) are the most numerous compact binaries in the X-ray sky. Yet their space density and luminosity function, hence their contribution to the total energy output of the Milky Way at high energies is virtually unknown. This situation will be fundamentally changed with eROSITA. Since all types of CVs are X-ray emitters, large volume-limited samples will be constructed for the first time. Based on these we will test current population synthesis models which predict abundant low-luminosity objects. We will quantify the incidence of magnetism on CV evolution. The contribution of white-dwarf accretors to the resolved phase of the Galactic Ridge X-ray Emission shall be determined.