In order to achieve the flow of current, electrons must flow from anode to cathode where these electrodes are immersed in a suitable electrolyte.
In case, a circuit is formed connecting both the terminals ( positive and negative) directly, the resistance of the connecting wire would be negligible and by applying Ohms Law, which states that the current within the system is directly proportional to voltage difference within the circuit and the flow of current is inversely proportional to the resistance of the wire.
Mathematically we can say that C = E1 - E2/R where E1 is equal to the positive voltage and E2 being the negative voltage. In the abbreviated form, it is defined as C =E/R where E = net voltage within the circuit and R corresponds to resistance of the connecting wire.
If the electrodes are joined together, the resistance of the circuit will be negligible leading to high flow of the current within the circuit and the magnitude of the current will be so high that it will damage the connecting wire and there are even chances of damage of battery. In order to avoid such situations, circuit breakers are used which will arrest the erratic flow of current with the breakage of the low melting wire.