The net field inside the capacitor and the potential difference across the capacitor is decreased when the electric field from the charge of capacitor plates are cancelled by the electric field from the polarized dielectric.More charge is needed by the capacitor to return to its original potential difference.
The net effect of the dielectric is to increase the amount of charge a capacitor can store for a given potential difference. The whole point of using a capacitor is to store charge, so coming up with a way to store more charge for the same amount of effort is a good .
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