There have been so many times in my "IT guy" history I would have loved to use my laptop's screen as a monitor for another computer. It's kind of crazy to have to track down a monitor just to see if a headless desktop is booting properly so I can remotely access it from the laptop that's sitting right next to it.

And my current MacBook has a retina screen, compared to the 1440x900 screen on my desktop. I'd LOVE to use my desktop's power on my Macbook's screen.

Non-hacky Thunderbolt 3 eGPUs like the Razer Core do offer a loopback mode where the video feeds back to the laptops screen over the TB3 cable.

The extra traffic on the wire does incur a small performance penalty over an external monitor though.

A few Sony Vaio models had that feature circa 2011 (HDMI in to display on the 16" WXGA (?) 1080p screen). I was very close to getting one.