A large current is needed to start the motor. The key is used to control the starter motor, when the key is switched on, if a large current flows from the key
to the starter. The key must be large and bulky, capable of withstanding the frequent passage of large currents without burning, and thick wires must be used to
connect the battery, key and starter motor.

It's not cost-effective, it's not reliable, and it's not easy to maintain. People invented the starting relay, let the starting current from the battery through the relay
directly to the starting motor, at the same time, must accept the control of the key. The control current through the key is small and the wire is thin, increasing the life
of the key.