I have the EOS R and I use the Fv mode almost exclusively.
What does it do?
Fv "Flexible Priority" mode lets you adjust aperture, shutter speed, ISO sensitivity, and/or exposure compensation without leaving the Fv mode.
In Fv mode: you turn the "Quick Control Dial" (the back wheel) to move the cursor between the four different settings. With the desired setting targeted, turn the "Main Control Dial" (the front wheel) to adjust the value of that setting. You can have all four values set to AUTO (which is equivalent to P "Program" mode), or have all four values set to a specific value (which is equivalent to M "Manual" mode).
What's so good about it?
The main benefit comes when you're using custom shooting modes C1, C2, and/or C3. The custom shooting modes allow you to save virtually all of the camera settings into three different profiles. (See Ken Rockwell's EOS RP User's Guide for advice about custom shooting modes.) But often you'll want a custom shooting mode to be defined by many settings other than what aperture, shutter speed, ISO, or exposure compensation to use, because these fundamentals usually vary from one shot to the next. The solution is to save Fv "Flexible Priority" as the mode to use in your custom shooting profile.
Now when you switch to, say, custom shooting mode C1, you'll be in Fv "Flexible Priority" mode so you can very easily adjust the shot to widen the aperture, or speed up the shutter, or add exposure compensation, or fix the ISO at 100, whatever the needs of your current shot. Without this Fv mode, you'd have to save Av mode or Tv mode in your C1 slot, and that would severely limit how you could use your C1 slot. If you saved Av mode into C1, and start shooting but then require a certain shutter speed, you'd be forced to jump out of C1 and into Tv mode (or a different custom shooting mode where you'd set Tv as the mode).
Fv "Flexible Priority" mode allows you to define custom shooting modes by everything other than aperture, shutter speed, ISO, and exposure compensation. And the EOS R and RP have a huge number of settings you might want to define (image size, JPEG/RAW, picture style, white balance, flash, viewfinder layout, eco mode, minimum shutter speeds, acceptable ISO values, etc), so the C1, C2, and C3 modes make it very easy to have three very different shooting profiles ready at hand. Fv mode just means you can have all of this without worrying about whether or not a custom mode will limit your control over the fundamentals of aperture, shutter speed, ISO, and compensation.
What's not so good about it?
The only caveat I've found so far (on the EOS R at least) is that Fv mode will totally ignore any "minimum shutter speed" value you've set (even when you have shutter speed set to AUTO in Fv mode). This makes it less desirable for an action-oriented shooting profile (where you'd typically want to set a minimum shutter speed of 1/250 or 1/500 of a second to avoid motion blur).
I can't think of any reason why Fv mode can't adhere to a specified minimum shutter speed, so I'm hoping Canon will address this eventually. (Disclaimer: my EOS R is at work at the moment, so I can't check whether this has been fixed by the recent firmware updates Canon have released for the EOS R. And I don't know whether it's a problem which exists on the EOS RP.)
I'm mostly using the EOS R for architectural photography, so the minimum shutter speed oversight is virtually irrelevant in that arena. But for a portrait photographer or sports photographer, this limitation in Fv mode might be a showstopper which forces you to save Av mode (which does obey any minimum shutter speed setting) into your custom profile instead.