Because the main purpose of Power Relay is to control the strong electricity with weak current. (including low voltage control high voltage, small current control high current). The effect is achieved by controlling the on and off of its own coil to turn its contacts on and off. (Popular point is to control the magnet to suck or not, and the magnet to control the contact switch)
Therefore, the connector of the relay includes the contact connector of its control circuit and the connector that controls its coil.
If you still have any questions, please read the instructions carefully, or the relay itself, we are Power Relay Supplier, can help you solve all doubts.