You have more than one Mac. You've got some songs in iTunes on one machine and some songs on the other, and manually copying the songs between them every time you add a new album is a pain in the *bleep*.
Even if music's not your thing, you've got folders on all of your Macs and you want to keep all your data up-to-date everywhere. Filesharing is too hard to set up, and it doesn't keep your data up-to-date automatically.
Enter Martian SlingShot. Just use SlingShot to Publish your stuff from one Mac, and then use SlingShot on your other Mac to Subscribe. Tell SlingShot how often to check for updates, and you're all set...SlingShot will silently work in the background keeping all your bits in line.