Why, you ask? Because the Fibersphere works with all fibers, including bast linen, hemp, cotton fibers, keeps the yarnball neat and clean, is easy to change without cutting the yarn, keeps me organized for subway, bus or train knitting in public and is semi-transparent so I can tell when the ball is running low to change to the next ball. Besides, it reminds me of my gerbil's ball in my '70's Habitrail set.
I've relied on a metal gauge for a few years, and discovered that my Addi Turbos were NOT the same gauge. In other words, I needed the Addi gauge for that brand of needle - other gauges were not accurate.