You are conflating a couple of things.
Adding a high pass filter to your mains does reduce the load on the amp, and does make them easier to integrate with a sub, but the Parasound lets you do that without an external crossover. This is to me a big convenience and simplifying feature.
The alternative is you route everything through an external crossover, often in the subwoofer.
Another advantage here is you can use a DSP based EQ on the sub alone, without having your mid/treble frequencies routed through it, they stay pretty much pristine.
Adding a high pass filter to your mains does reduce the load on the amp, and does make them easier to integrate with a sub, but the Parasound lets you do that without an external crossover. This is to me a big convenience and simplifying feature.
The alternative is you route everything through an external crossover, often in the subwoofer.
Another advantage here is you can use a DSP based EQ on the sub alone, without having your mid/treble frequencies routed through it, they stay pretty much pristine.