Specific sound cards, video cards, operating systems, and memory managers were required. Lapsus by Maturefurk. A great example of artistic freedom and technical experimentation. Importantly, demos are not interactive. This allows a demo coder to make many optimizations unavailable to the game programmer.
Demos are much more like a technical sketch in that sense.
They allow novel concepts to be focused on, without the enormous amount of work that would accompany making a full game production. Additionally, demos are artistically free. They need please only the authors. This empowers them in a way similar to many independent films. They provide a medium to explore, without the constraints of economics.