If a dielectric material is inserted between the plates of a capacitor the capacitance increases. In other words, the capacitor can hold more charge before reaching a given voltage if a dialectric is present.
Polar materials make good dielectrics. Why is that? The polar substance acts to weaken the field between the charged plates.
How does a dielectric substance weaken the field associated with a given amount of charge on the plates? This occurs because the electrostatic force from the external field causes polar molecules to rotate and orient themselves opposite to the externally applied field. This produces a weaker overall net field with the individual dipole moments of the molecules in opposition to the orientation of the field.
Because the field is weaker with the dielectric present, the voltage between the plates is lower than it would otherwise be. In other words, more charge is now going to be required to produce the same voltage as before. The dielectric between the plates has increased the capacitance of the capacitor.