Two reasons. First, to accommodate the hanging rail for the Camar Suspensions System, but even if you choose not to utilize the Suspension System, you will still have this 10mm space we like to refer to as Back Scribe.
From the name, you have probably already guessed the second reason for the 10mm space behind the back. Building are almost always framed with wood, and walls made from wooden studs are not only seldom plumb, they are seldom straight (they have humps and bows in them).
The 10mm Back Scribe gives you some breathing room to scribe cabinets to bad walls when needed. And on occasion, you will find it useful for the plumbers loop de loop trick to not run drain pipes through the bottom plate of a wall, this extra 10mm of space might be just enough to keep you from butchering another of your awesome cabinets.