A more traditional answer is that if you purchase and use external amplification you can shop for the best amplification for your speakers that is available, and then you can use a receiver/preamp which is the best, regardless of internal amplification.
For my needs, I struggle, because I personally use an external audio/video switcher for all of my stuff and all I need my receiver to do is decode digital audio and amplify it. If I went with a dedicated external amplifier of good quality, I could then buy an inexpensive A/V receiver with the latest audio decoding, but maintain the top shelf amplification.
I think mostly it is for people with big speakers and big budgets who really want to get that last 5% of quality out of their audio, regardless of price. On the practical side, there is less there because of the current pricing of A/V receivers.