GHD also is known as the Glute - Hamstring Developer is one of the most underappreciated pieces of equipment.
The two most common movements you can do is a sit up and back extension, but so much more can be done. It's calling is to help build your glutes and hammies so the GHD Raise is where it's at.
Why do I want to use this machine?
The "posterior chain" is targeted by using this machine, which you may or may not know as the group of muscles on the posterior of the body including the hamstrings, glutes, erector spinae, trapezius, and posterior deltoids. These muscles all work together to allow you to perform basic movements like running, jumping, and squatting, which are all regularly used in weightlifting and sprinting.
Like most accessory exercises, using the GHD can not only help you improve your main lifts but also help with preventing injury and strains.
Here is a video that demonstrates some of the most common exercises you can do with a GHD.