Why does OpenGL require all coordinates in [-1,1] (NDC)?

Question

Why shouldn't the z coordinate be normalized to the [0,1] range instead of [-1,1]? As I understand it, that happens some time after anyways, so what's the catch with this?

Answer 1

The question could very easily be turned around: why the [0, 1] range? What makes that range special?

NDC space is a cube; every component is on the [-1, 1] range. So it's very nice, neat, and uniform. That's why OpenGL uses that space. It's simple, obvious, and very regular.

What you suggest is to turn that nice, neat, uniform cube into a rectangular prism. There is nothing to gain from such a space. Why be irregular when you can be regular?

As I understand it, that happens some time after anyways

No, it does not. The transform from NDC space to window space uses the current glDepthRange setting. That may be [0, 1], and that's what it is by default. But there's no rule that it must be. It can be anything you want, so long as both values are between 0 and 1. You can even reverse the near and far, using a range of [1, 0].