My application is straightforward. Noise Reduction gracefully removes hiss and background noise from vocal recordings and Noise Removal doesn't/didn't. If the user is careful about the Profile step, we can simulate a microphone with a lower noise floor than actually exists.
My go-to tool is the Noise Reduction of the Beast: 6, 6, 6, with a variation of 12, 6, 6 if the presentation needs a little more help. If more help than that is needed, I usually go straight to fixing the studio or microphone rather than trying to "clean it up" in post production.
6, 6, 6 is the difference between a prominent hiss level behind the performance and a hiss level that you can hear only if somebody points it out to you. Otherwise, it vanishes, even though it's only a 6dB change. It can also put you over the edge if you miss ACX Noise by juuuust this much, and vanish while it's doing it.
It's pretty impressive and I had to sit down the first time I found out it would do that.
"Oh, this is good. Let me write this down...."