Mostly I use the high digital octaves for parts where the bass and guitar double each other but our only guitar player is doing something else, such as "Don't You Care" by The Buckinghams. I also use it on some songs where our keyboard player has developed this annoying habit of stopping playing keyboards and coming out to front songs because Steve Walsh from Kansas does it. A little goes a long way but he's trying to do it on every song he sings

Usually I use an EHX Pitch Fork, though, which is much better. The Mooer is smaller and doesn't require an isolated power supply, so I've been trying to make it work on fly gigs.
Low octave is mostly for Hollywood endings, and I usually use the OC-2 exclusively for that unless I'm flying, although I do slip it in sometimes for occasional songs from the 70's that I think could benefit from having a little synth bass tone.