I’ve read that part of why GIMP is the way it is is because it’s meant to be a testbed for the GTK UI library, so features are added to use new UI elements as much as they are to aid photo manipulation, and in some cases it’s considered preferable to use a weird widget so it’s got a test case rather than whichever widget leads to the best UX. I don’t think I’ve ever looked for a more definitive source than a Lemmy/Reddit comment, but it’s at least consistent with my experience of using GIMP.