The octave-down thing is just the original vocal double at its original pitch. Emphasizing that (or the original unaltered lead) in the mix and using the "female" voice as the backing vocal provides a very different effect, more of the Cohen/Cave/Sisters of Mercy/etc. thing. Which was the original intent, until I realized reversing their roles could be amusing. It's Melodyne's formant shifting that makes it, with some manual tweaks to certain notes, passable. Now, if I could figure out how to remove the metallic modulation artifacts, I might actually use it as a lead voice on a thing or two.
The formant tweaking is what makes it useful- you have to shift the formant up a bit aside from just the octave, and it's very much a "by ear" thing to correspond to an individual voice. My limited fooling says it's better for turning a baritone into Robot Stevie Nicks than turning a tenor into something that sounds like a cartoon character, and it prefers generally cleaner, less gritty singing.
Future boredom will likely see further examples of this foolery. Mostly, I think, to create backing vocal layers that don't sound horrid soloed. But for the odd thing or two, it could be an interesting alternate voice.