Why do electrochemical cells need a salt bridge?
The salt bridge is needed to assure the neutrality of both compartments.
Electrochemical cells, galvanic or also called voltaic cell cannot run for long time without a salt bridge because the cathode and anode compartments become charged with time and the attractive and repulsive forces will prohibit the flow of electrons within the cell.
Thus, the salt bridge is needed to assure the neutrality of both compartments.
The cathode compartment (where reduction occurs) becomes more negative with time, and the anode compartment (where oxidation occurs) becomes more positive with time.
The salt bridge therefore, is used to pump positive ions (cations) in the cathode compartment and negative ions (anions) in the anode compartment.
Here is a video that explains this phenomenon in more details.
Electrochemistry | The Galvanic Cell.