Navigation should include option like "remember position on page" when moving to next/previous page. Something like xdvi does. This would permit more flexible navigation. Reproducible: Always Maybe this option exists, but in that case I have been unable to locate it (documentation problem)
How do you move to the next page? When I assign the PgDn key to the "Next Page" action in Settings > Configure Shortcuts, then it does exactly what you described. You can use a different key, of course.
Maybe I have overlooked something but "Next Page" moves to top of the next page AFAIK. What I mean is to move to the next page but keep the position. Say, you are in the middle of the page (and the page is larger than your screen). Now pressing "Next Page" throws you to top of the next page, not in the middle. What I have in my mind could rather be called "Page right", not "page down" where you imagine that all your document is horizontally spread on a large desk...
What's the use case of this?
It is a way to navigate the document in 2 dimensions (up-down, left-right), instead of one dimension (up-down), so it speeds up navigation. For instance, if I have to switch between 2 pages and look for something in the middle of the corresponding pages, there are good changes that I can achieve this by just pushing "next page"/"prev page" a few times. This is also rather intuitive. One can imagine the pages are set next to each other on a huge desk, and we move the window (computer screen) left and right across this sequence of pages. If one lands on top of the following/previous page, then the intuition breaks down. Moving only in one dimension (up-down) takes more keypresses and is harder to do precisely (easy to over/undershoot). However, I have never done any market research and I don't know how much demand there is for such a feature. It exists in xdvi and mupdf.
I have another, admittedly probably quite rare, usecase: I have an animated map in pdf (each page in the pdf corresponds to a frame in the animation); each page is very large and one has to zoom in to see details. The suggested action would allow me to use okular to view the pdf as an animation by repeatedly invoking the action.
*** This bug has been marked as a duplicate of bug 169614 ***