I rename mine Local and use it to import ONLY files that live on the C drive. I hardly ever use it, but it is occasionally useful to have a way to use MC on random media files you have on the C drive, even if most of your media lives off on a separate drive elsewhere.
Every so often, I update the "framework" of the Local library to match my "real" library. This is easy:
1. Make a backup of your main library, and save it somewhere easy to get to, like the Desktop.
2. Switch to the Local library.
3. Restore the backup. DON'T check the "restore settings" box, as you don't need that part.
4. Switch to the "all files" top-level View I made for file management (if you don't have one of these, you really should).
5. Select All the files. Delete, and choose "Remove from Library" (DO NOT choose any of the other delete options, as that'll delete your files if it can get to them).
6. Setup Auto-Import to watch the whole C drive (or whatever subset you want it to include).
7. Switch back to your regular Library.
It is a handy tool to have, occasionally.
