The preference menu in Shotwell allows the user to specify both an 'External photo editor' and an 'External RAW editor', but I'm confused as to why two external editors would be required. I'm not a photographer, so this confusion may simply be a result of my ignorance, but I thought RAW images were unprocessed photographs, in which case two editors would be kinda redundant. Am I simply missing one of the finer details of photograph processing?
You are correct about RAW files being unprocessed shots. In fact, they're so unprocessed that the files are not even JPEG files. Thus RAWs are typically converted to JPEG. For this, you would typically use a RAW Editor. A RAW editor usually does things like handle tint, exposure, things like that, which can be best tweaked before it converts to JPEG. This is the basic task of a RAW editor, such as UFRaw or Rawstudio. They give you control over the way the data from your camera's sensor becomes that JPEG file.
An external photo editor on the other hand would be a program typically for editing regular JPEG files, and usually offers a lot more things you can do to the photograph, such as airbrushing etc.
So why are some types of things expected to be done to RAW files while others are reserved for JPEG? Well, I don't have a good reason, except that's the way it's always been done. Programmatically, the task of converting to JPEG is quite different from the task of editing the contents of the image, even though an average person such as yourself would see the two tasks as essentially the same. Personally, I would love to see a program that let you edit RAW files as if you were editing a JPEG, and allow you when finished to save the finished product as a JPEG.