Reason #1: Apple is seeing growth in sales of the Watch, and plans to give it cellular capabilities will drive growth further. If cellular networks allow you to share your number across your phone and watch, it could really support people who may want to go without their phone from time to time.
Reason #2: is my belief that the Watch will provide Apple with a platform to develop miniaturization. Having different platforms and different scales allows Apple to innovate and gets their partners like Intel to do the same. This will support Apple in being either first or best in the areas they have products.
I'm glad to see the Watch continue to grow. I would not be surprised to see some of the features we once saw in the Shuffle morph into the Watch. That would make up some for the lost of that other once great platform of Apple, the iPod, which was loved and now lost.