Editing features overlap between Rawtherapee and GIMP

[Ofnuts]

RT (and others...) is a lot more photo-oriented than Gimp. They are also non-destructive, they store the process, not its result, so

• you can figure out what you did,
  
• you can go back partially in the process history to change the process without redoing everything
  
• you can easily apply the same process to more photos (for a panorama, for instance)
  

To import a raw file, you will always need plugins (importing any file type in Gimp is done by a plugin). So the only question is how complex the plugin should be. RT is a full application masquerading as a plugin, there are simpler ones (NufRaw...).