Slip Ring Induction Motor

Because there is a fault. Get it checked before re-energising. If the facility can't do without a motor installed at that place, install a spare.

Or is it an anti-condensation heater? Without access to the wiring diagram, the terminal box or the block cable diagram, it's a bit difficult to see from here.

I believe OP wants to know why the external resistance on a wound rotor induction motor heats up. From Wikipedia:

"...When used with a load that has a torque curve that increases with speed, the motor will operate at the speed where the torque developed by the motor is equal to the load torque. Reducing the load will cause the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor torque are equal. Operated in this manner, the slip losses are dissipated in the secondary resistors and can be very significant. The speed regulation and net efficiency is also very poor (due to the losses in the secondary/external resistance)..." Comment and highlighting added.

Maybe it's because the motor is not 100% efficient.

Please clarify for the discussion. Slip ring connection to the rotor is used to carry DC magnetization current to the rotor to generate a well regulated magnetic field.

Slip in a asynchronous motor is the measure of the rotor being slower, than the rotating magnetic field.

Which one is it?!?

Sorry, no to the first comment.

Generally slip rings are used to carry/transfer current/voltage/signal from a stationary part (brushes) to the rotating part (rotor), or vice-versa. Specifically in the case of a wound rotor induction motor the slip rings allow the insertion of an external resistance into the rotor, the current in the rotor is AC, not DC.

Check this animation out.

The resistors are normaly only in use during start up as shown here. The heating will be for a short time.

In the old days before VFD's and the like, the resistance was also used as a means of speed/torque/overload control. It is very inefficient because of the high losses in the external resistance when the rotor speed is held significantly below synchronous speed. See #7↑