When sat on a desk, the Retina iMac is an imposing glass wall. The desk space behind it, a rumor. I like having easy access to the top of my desk. It’s where I can keep pencils, pens, notepads, headphones, teacups, and other objects that would fall to the floor if they weren’t sitting on a solid surface. With a floating monitor, I had comfortable access to my desk, but with an iMac on a foot, I really didn’t. (Also, a monitor on an arm is far more adjustable than one on a desk—extra useful when you’re using a sit/stand desk.)
At that moment I realized that this meant I was going to be ordering the VESA-mountable version of the iMac. You may not know this, but up until the most recent generation of iMacs, you could mount any iMac on a wall or arm by popping off its little metal foot and screwing on a VESA mount adapter. Unfortunately, those days are now over: When you order a new iMac, you can choose a VESA version with no foot at all, or the normal footed version that will never be mountable.
I'd seen the VESA-mount option in the Apple Store but never really gave the why of it a serious thought. Seeing Snell's setup, now I get it.