There are lots of reasons that someone might want a head-mounted display. Camera operators and radio-controlled vehicle enthusiasts typically like these because they keep the sun off of their screen while working outdoors. Aside from those practical purposes, strapping a high-definition display to your head is just cool. Add some motion sensors to that and you’ve got a homemade Oculus Rift Virtual Reality display!
The team at Adafruit has released plans on how to make your own. While their plans do require a 3D printer for some of the case construction (a fancy multimaterial capable one too!), you could make the case using Google Cardboard.
The electronics involved are fairly simple, consisting of a screen, a 9-DOF IMU board, and an Arduino. You can find the schematics and code on their site.
Regardless of why you may want to make your own head-mounted display, we can probably all agree that Noe Ruiz is wearing an awesome hat in that video.