Well, thinking about this logically, projects and tasks cause a mess and/or the actual project has minor details to perfect it. When you're finished, you put away your tools and then dot your i's & cross your t's. When something is "done and dusted", it's not merely created or accomplished, it's also polished and cleaned up after. Nothing else is needed, so it can be considered "case closed".
I had a lot to do last week, but I got it all done and dusted. Today I relax! (... This will never be said about a Microsoft product.)