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.

You can do interesting things in the scroll bar. Some coding editors (like Visual Studio) cram a lot of useful information into the scroll bar.

https://learn.microsoft.com/en-us/visualstudio/ide/how-to-tr...

For mouse users, clicking and dragging the scrollbar is the fastest and most intuitive way to scroll through a large document or list. (The scroll wheel, if you have one, is much slower.)