These paths determine the folder in which Sketchup starts - e.g. when you say save_as a component, it first opens your 'default' components folder...
You set these up for various 'objects' that you might want to 'export' out of your model, it's just a way of simplifying your navigation to the likely destination folder.
If they have never been set [or the folder-path is no longer valid - e.g. you've renamed a folder] then I think that Sketchup assigns your Documents folder as its initial default, and of course you may well wish for your collection of saved-components to be somewhere entirely different...