Here's an example: say you have JTR 215RTs, and a sub. Normally most would suggest using an 80hz crossover. That will just waste all the potential you paid good money for with the 215s. Instead you could run the 215s full range and still send everything below 80hz to the sub (along with the LFE). That gets you 80hz and below output from all the speakers, not just the sub. The JTRs can handle anything an avr can send to them, but the diysg kits may not be able to handle 100 watts at 20hz so they need some form of high pass.
The diysg kits that have capable woofers can play pretty darn good down to 40-50hz. To demonstrate this, setup your system with an 80hz crossover and unplug the sub. So only the mains are playing with the 80hz crossover still on. You will not hear much of any bass out of them. Now, with the sub still unplugged, turn the crossover down to 40hz. I bet you enjoy the sound of them more with the 40hz xo. But with a 40hz xo, the sub is now wasted since it isn't playing anything above that.
Instead I'm suggesting we let the mains play down to whatever range they can safely handle, and use the sub up to whatever range works best. I'm not aware of any avr that has this type of feature, probably because they assume we want a flat response and the crossover as designed is ideal for that. But we like our bass hot, and just turning up the sub level creates a huge cliff between the mains and sub. Letting the mains and sub overlap in the 40-100hz range can soften that transition and add some additional midbass.
With pre-outs, you can send the mains the full range signal via LFE+mains, and then just use the outboard amp to set the high pass. When using only an avr you need some other form of high pass to protect the woofers, which is what I'm after.