If you don't understand why this would be useful, that's fine. But you don't have to use such a feature either. They are merging RT & WP as quickly as possible. They are bringing the ability to project to other screens and even have them be "screen aware" which means you can open apps into a different screen, and still utilize your phone like normal (similar to a Win8 tablet/computer). The ability to also run stationary desk items such as a keyboard & mouse to your phone and utilize your phone as a pocketable, super portable computer is definitely useful. I look forward to being able to do things like this without needing to bring other devices into the mix.
I would love to be able to be at somebody's home, slap my phone down on the table and use their peripherals to quickly do some work or even play a game off my phone which is being displayed on their TV or monitor. There are tons of possible uses for this, and it just adds to the devices capabilities and usefulness. It doesn't mean you have to drag a keyboard/mouse with you at all times, it means when/if you could use them, you have the option.