Figure 1: Torque-speed curve for asynchronous motors, with important regions designated.
The squirrel cage is a simple method of pulling an electromagnetic induction effect.
So, that current flows through that.
They are particularly suited for applications where the motor must maintain a constant speed, be self-starting, or there is a desire for low maintenance.
Prior to widespread variable frequency drives, induction motors of medium and high power were made with a wound rotor.