What is the purpose of a scrollbar? I don’t see the need.
Suppose you're working on an interface to show something that's 2D, is generally 3× as wide as the viewport and 5× as as high, and the viewport is big (a computer screen, not a smartwatch). In that case two scrollbars may be the best way to let the user navigate. The user can see at a glance what part of the thing is visible within the viewport, can move the viewport by manipulating the same thing that shows the location, and the movement has a low error rate (the effect is generally close to the user's intent).
If the typical sizes are, say, 100× as wide as the viewport, then the scrollbar doesn't work as well. If the viewport is so small that the scrollbar takes up much of the space, ditto. If the viewport is a touch screen, that changes the tradeoff compared to a draggable viewport.
Namely, it's about the question, whether the scroll thumb should be flexible in size, thus representing the size of the viewport in relation to the total dimension of the document, or not. If it is, the user can obtain information about the approximate size of the entire document nearly immediately. On the other hand, if it does not, it rather emphasizes the position of the viewport in a document and allows for more precise navigation to a certain location in the given document. Moreover, this second type of scrollbar doesn't suffer from issues with very long or very short documents. (As the second option provides a bit more usability, it was quite naturally removed in the process, we know as the history of user interfaces.)
When keyboard is not supported and I can't scroll to the bottom.
When I want to scroll to find something in a binary like fashion, scroll to the middle, decide if what I want is at upper half or bottom half, etc.
Not all people have scrollwheel to begin with
Scrollwheel is broken, as many overwrite OS behaviour, and generally slow even if you specify in settings to perform larger motion per scroll action
most scrollwheels have "notches", so if you move the wheel one "notch", its rarely the exact amount you wanted to move. the middle click is also not ideal, because it starts moving the page based on the current mouse position relative to the original position. so if you accidentally move too far away from the origin, the page shoots up or down.
with a scrollbar done right, you have fine grained, manual control of the movement of the page.