It seems the main purpose of Flat Field is to correct for vignetting for various lens/aperture combinations, as well as sensor dirt, but it also works well for photographing 2D artwork, especially large pieces. Recently I was commissioned to photograph a historical poster for a client. I set it up with 2 LED photo lights at 45 degrees, each light covered with polarizing film, and the macro lens also with a polarizer. This combination eliminates any glare on the art.
Even with the lights carefully placed, the light on the piece was still slightly uneven. I shot it again, this time with a clean piece of white foamcore underneath. I then removed the poster and shot just the white foamcore, but with no changes to the lighting or camera settings. I used this second shot as a Flat Field in Rawtherapee. Worked perfectly.
I think this would be especially useful when shooting painting with a texture that you want to show. In this case you only use one light, maybe at a 20 or 30 degree angle to the painting to show the raised texture. With just one light there will obviously be light falloff on the side further from the light (unless the single light is far enough away, i.e. the sun!). Using a piece of white foamcore as a Flat Field shot should eliminate the problem.
I continue to be impressed by the features and quality of RawTherapee. Kudos.