You obviously never used one. It gave you full, immediate control over how fast you scrolled, which buttons could never do. It enabled you to immediately change direction without lifting your finger. You could start by placing your finger anywhere on the wheel, which made it easy to use in the dark, and moving your finger from the wheel to the button in the center was just as easy.
It was, and still is, the best interface I've ever used for navigating through large, grouped lists.
You could also adjust the volume through your pants, at least with some versions. It was just sensitive enough that I could do this with jeans without having to stick my hand in my pocket. Honestly this was one of my favorite features, at least until the headphone remotes.
It was, and still is, the best interface I've ever used for navigating through large, grouped lists.