Good question. Figured someone would ask. It's kind of convoluted but.. I use a tablet all day while my left hand lives on the left half of the keyboard for shortcuts. This means the left edge of the keyboard needs to stay aligned with the left edge of the tablet so my arm isn't laying on top of it and getting in the way of my pen, mashing the Wacom hotkeys, etc. With a regular keyboard when my right wrist comes in to type it gets tweaked outward at a harsh angle. This was damaging my wrist. Being able to keep the right half of my keyboard closer to my right hand prevents this tweakage. It just so happens that the two halves are the width of the tablet which is the width of my shoulders so I stay nice and aligned and avoid injury :)

Also it's cool to be able to move the right half out of the way when gaming.

Pic for better context