What is difference between Power-GND vs Pre-GND?
Most likely the power amp ground has significantly more current on it than the preamp so they provide separate pins so that you can run separate ground wires back to a single point at the power supply next to the power supply filter capacitor(s). Otherwise, if you just connect the two grounds together at the IC, the inductance and resistance of the one ground wire (albeit small) will cause non-zero voltages which will be reflected on the preamp ground. Meaning you could get increased noise or possibly instability or some other equally bad result.

Grounding issues are the most common cause of noise and hum in audio circuits. The solution in virtually all cases is to have separate ground wires running back to a point where ground currents are no longer present (b/c they have been absorbed by filter capacitors).

For example, if you have analog signals and digital signals in the same circuit, it is absolutely vital that you have separate grounds. If you don't, you will get digital noise in your analog signals.

Another example is shields of pro-audio cables which are (supposed to be) connected to the chassis (at least from the driver end of the cable / receiver end should not be connected at all). Then the chassis is connected to the power supply ground origin (meaning at the filter caps).