The capillary tube has a much smaller cross-sectional area than the other pipes in the system, thus causing a restriction in the passage of the refrigerant fluid. This restriction ensures the maintenance of the condensing pressure in the condenser and causes a pressure drop in the fluid at the evaporator inlet, bringing the pressure from approximately 135 psi to approximately 9 psi.
This pressure drop in the refrigerant is important because it takes it, passing through its saturation line, to its mixing zone where it will absorb latent heat from the refrigerator's internal environment so that it can vaporize.
