1. More hand positions and more options for fingering
2. Ability to play more around 5th to 7th frets
3. Options to avoid open strings and even out attack and sustain
4. A few lower notes -- nice to have a low D without detuning
5. Drives guitarists nuts trying to figure out what I'm playing
For all of these reasons plus:
6. Changing keys is a breeze as you can play everything without using open strings.
7. You can cover 2 full octaves within 5 frets (playing across the strings)