When you're recording, you can use all kinds of exotic instruments for embellishing sections of a song. But you probably would like to be able to recreate your recorded performance live. It would be cool to be playing something on a six-string and have certain notes of the melody jump out with a 12-string sound and attack.
With a properly set-up Jazzmaster/Jaguar-style bridge system, you can down-tune from standard tuning, and the guitar will still be in tune, and, as a bonus, you can still use the tremolo bar, though only go down in pitch when down-tuned. This also can allow you to keep playing if you break a string in the middle of a song.