For me? Because I can run all the apps I make a living from - Photoshop, Zbrush, 3D Coat flawlessly, as fast as I can on my iMac 5K - the quad-core Skylake i7 will handle everything with ease, and the super-fast SSD will help me open those 500Gb PSB files faster.
The screen seems to be amazing, bright - important for illustration work. I'm not saying the P3 color gamut is required, but I'm certainly excited to use it.
I can use all of my pro tools on it and have more ports available - I can attach my Intuos Pro (with a cheap adapter), USB-C lightning cable for my iPad Pro (to use Astropad via cable) and still have 2 USB ports available - for my mouse (on my current MBP I have an issue when I connect the Intuos on one port and the iPad Pro via cable - on the other, I have no USB ports left). Even better, when I buy the new Cintiq Pro next year, I will be able to connect it directly to the USB-C port and use one cable for power, usb signal and image instead of 3 (yes, 3!) I needed for my Cintiq Companion (that one device used up half the ports on my previous MacBook Pro, and is one of the reasons I stopped using it).
That's a great example how USB-C/TB3 is helping professionals. The Cintiqs are nothing but professional devices - they are expensive tablets used only by people who make a living with them. And these devices require 3 thick cables and a power brick (I'm not kidding, all these cables and the brick require a separate bag for me!), and now they can use just one, simple, thin cable - and no power brick as the laptop will provide power. Great times.
Also, even with all this desktop-class performance, I will be able to easily carry around this computer around, not only between offices in my studio (the great battery life will help, as I won't have to search for power outlets as I visit each team) but when I work from a different location completely. I currently use my iPad Pro to discuss concept art, but I cannot open ztool and PSD files there. And my current MacBook Pro is a bit heavy, the difference is not big, but you'd ve surprised how many times I lift it and just leave it on the desk, grabbing my iPad. I don't know if that will change with the new MBP, but I do know that I carried the 13" around much more. It's not a necessity, but it is a convenience to have such a portable, and yet a powerful computer.
Finally, I believe the Touch Bar will be useful in Photoshop - I won't have to rely on my Wacom Remote to have a capacitive slider to change the brush size, it will be built in, alongside other options. One less thing to carry around.
In fact, this is something of a dream computer for me. It's how I imagined computers of the future, when I started doing digital work.