said by tired_runner:One would want to root an Android phone to unlock features that carriers typically lock to charge customers extra for. Rooted phones let you install different ROM's that often run much better than stock and without the carrier-issued bloatware.
Example, not paying for what use to be PAM service (phone as modem) and "tethering it" to a laptop so you can surf the net at will. Or, making it a wifi hotspot, (another charge) allowing freeloaders to use your phone as a router or allowing kids access while driving down the highway so they can play games online.
It's akin to removing macro-vision from a movie so you can play it through certain devices or make a copy. In a nut shell, it's for avoiding payment for services that big cell likes to charge you for.