Funny you should ask this because last evening I FINALLY got my head wrapped around this mode and who it's best for. I'll attempt to explain.
To me, it's like a hybrid of M, Av, and Tv modes. Referring to the R6 (because that's what I use), you turn the Quick Contol Dial 2 (top thumb dial) to switch between shutter speed (Tv), aperture (Av), ISO, and exposure compensation and then change each setting with the Main Dial (top index-finger dial). I guess the main advantage of changing these in Fv mode as opposed to M (manual) mode is the ability to set each one to AUTO at the press of the trash can button (hold it to set all 4 to AUTO). In M mode, the only setting you can change to AUTO is the ISO, but you very annoyingly can't do it at the push of a button like in Fv mode.
I guess another main benefit of Fv mode is not having to turn the mode dial to M, Av, or Tv modes. You can stay right where you are.
I like M mode best because I can change Tv, Av, and ISO with 3 separate dials, not having to first switch to one of those settings before being able to change it as you have to in Fv mode. However, being able to quickly change any setting to AUTO at the push of the trash can button makes Fv mode much more compelling to look into. I might use it more since I get incredibly annoyed at how you can't reset ISO to AUTO in any other mode without having to spin the dial all the way to AUTO!
I guess I've only tried to explain how it works and how it differs from M mode. As to the best situations to use it for, I'll leave that for somebody else who's actually used Fv mode extensively!