It's not for everyone, but many people do want the choice to have it. The main reason for example is if you're watching a video or playing a game, for example, and then you go back to the start screen... why should you have to rotate your phone vertically again? Instead you should be able to stay in horizontal mode if the user wishes too. It'll also bring consistent to the platform. All the built-in apps and settings work in horizontal mode, yet the Start Screen doesn't. Very minor, but very useful to have (for those that choose to use it)