Why salt bridge is used in a voltaic cell?

The salt bridge (filter paper soaked in saturated KNO3 solution) serves two main functions :

(1) It completes the circuit by allowing ions carrying charge to move from one half-cell to the other. In the absence of the salt bridge, it is an open circuit.

(2) It provides cations and anions which replace those consumed at the electrodes or which balance the charges on any ions formed from the electrode.

Is is used to prevent the build up of charge.