It's not about CMYK though - that's an even smaller gamut than RGB.
I had a look yesterday and GIMP eventually added color management capabilities. Good for them. Still don't like GIMP tho.
The first time I tried GIMP (after I got the cavemen out of my office) it was a direct RGB editor. You were painting directly into the RGB pixels. That's fine I suppose if you are editing icons for a computer screen - there's a place for tools like that.
The problem is that RGB color spaces, even the "large gamut" spaces you find on better monitors, laptops and Apple equipment represent a fairly small subset of human vision. Ditto the "Pro Photo RGB" gamut: it's bigger than the "standard" sRGB color profile gamut that's common in run of the mill monitors, cameras, printers, but doesn't approach human vision.
For a photo editing program, you need a color space that is as large or larger than human vision. You're doing your editing in that color space (though the human operating the software may not realize), and you're seeing an sRGB or other translation of the color on your monitor. Exporting the file to an RGB or CMYK (or other) file format fixes the image into one of those color spaces (and assigns or includes a color profile if the software and user are doing it right).
Most of this is transparent to the user (not the business of color profile, though: the machine can't usually guess for you)
Editing photos directly in RGB is the equivalent of a video production company (back in analog days) creating and editing videos exclusively on VHS tape. There's information loss every time you make a generation copy.
There's also information loss every time you make a digital transformation. Using a large color space behind the scenes minimizes that transformation information loss.