> It's interesting that the author talks about posterization at 8-bit, but demonstrates it with a 50-level gradation (i.e. less-than-6-bit). Why would someone do this?
Because you only get all 8 bits of range if your endpoints are black and white, respectively. The place where low bit-depth bites you is in subtle gradients; the closer the endpoints are, the fewer steps you have.
Consider, for example, a gradient from (0.5f, 0.5f, 0.5f) to (0.6f, 0.6f, 0.6f), or in 8bit hex terms, from $808080 to $9A9A9A. That's a 27 step gradient.
You also lose precision when you blend gradients between layers.