In an electrochemical cell, there will be positive ions migrating toward the cathode and negative ions migrating toward the anode. The salt bridge is used to maintain electrical neutrality in the system. The ions need to flow back and forth between the two half-cells. KCl or better yet, KNO3, are just simple, inert salts. KCl would NOT be a good choice if you had silver or lead electrodes, because it would lead to some precipitate formation.