When reading a document in a browser, I rely on the scrollbar to know things like: how long is it? Where am I in the document? How much of the document is on my screen right now?

This is critical for decisions like: "Should I read the whole thing?" and for building a mental map of the whole document.

I use the scrollbar to scroll between parts of the document if I need to flick back and forth quickly, say between the data and the interpretation, once I have that mental map and know where things roughly are.

While reading, I'm dragging or wheeling.