They are used for connecting amps to your reciver to power your speakers.
They come usefull because if in the future you want to upgrade to say M80s, they are 4ohm and may require a separate amp, in which case you can keep your same receiver and just buy a couple of amps and hook them up to your receiver through the preouts.
This would also apply to running parallel speakers like a centre channel, 2 VP150s run in Parallel would run at 3ohms and would require a separate amp.
Its just very usefull to prepare yourself for whatever comes in the future so you don't have to buy again. Finding a reciever with preouts these days is very easy, I am sure all the new Denon models that you are considering have them.
