Scroll lock mode (arrow keys scrolling the window rather than moving the cursor) is almost identical to the way the arrow keys work to scroll a web page – a sort of keyboard equivalent to using a mouse scroll wheel or 2-finger scrolling on a trackpad. It's particularly nice in that it scrolls a single row or column at a time, and particularly useful for spreadsheets (Excel) and text editors.
I currently have a compact ("tenkeyless") keyboard which lacks the key, so I tend to use a spring-loaded setup where holding down alt/option/etc. and an arrow key scrolls the window.
This is basically what the Scroll Lock key was meant for in the first place, but the convention never really caught on, so the continued presence of the key baffles a lot of people. I recall FreeBSD's console driver supporting using Scroll Lock in a similar way to move through the scrollback buffer, although I don't know whether this is still true in practice (e.g. if using a UEFI or drm console driver rather than old-school VGA).
I currently have a compact ("tenkeyless") keyboard which lacks the key, so I tend to use a spring-loaded setup where holding down alt/option/etc. and an arrow key scrolls the window.