There are 2 main advantages;
The 1. is the capability to bridge any stereo power amp.
Here it is important to remember that the load on the amp will be the half, this means that if the speakers are 8ohm the amp will see a 4ohm load!
The 2. is to distribute the workload on a power amp witch is having a little trouble driving the speakers hooked up to it. If you invert one side of the power amp (like the Left channel), then you can cut the workload dramatic due to the sine wave is working opposite to the other channel. When one side is pulling juice on the + the other is using -, of curse it is important also to switch the phase on the speaker so the sound waves coming from it is not inverted.
