Ic package ground plane

Inside an IC package, what is the purpose of ground plane. Is it connected to the backplane of substrate for biasing it.

Is it useful only in high frequency RF circuits or required for low frequency analog and digital circuits also.

I used an audio IC which had overly wide pins designated 0V ground. They were tabs about .4 inches broad. They required a long slot in the circuit board. They must have been for cooling.

For two purposes
-Grounding
-Heat