Well, i already mentioned an example: image editors. IMO image editing is one of the perfect cases for MDI because not only you can have multiple images visible at the same time, but also multiple views of the same image (useful for pixel art, for example). Most MDI applications allow you to dock and/or float stuff in the edges/over the windows which is useful to avoid repeating the same chrome at every window (which is what a lot of applications that support multiple documents with toplevel windows do).
In theory you can have multiple toplevel windows with separate windows for the control stuff (tool window, tool options, panels, etc like GIMP has) but in that case you really need a virtual desktop dedicated to the application itself. Personally i prefer to dedicate virtual desktops to tasks (i have a fixed number of virtual desktops and their shortcut keys have become muscle memory over the years), so e.g. anything graphical goes into the same virtual desktop, but -say- GIMP in multiwindow mode feels awkward to use alongside Blender. Krita having an MDI mode is much better IMO, even if Qt's MDI support is primitive at best.
