It was intended to give a way of keeping the cursor positioned in the same place on the screen when pressing up/down arrow and having the screen scroll up/down a line rather than move the cursor. Seems though it's not so commonly used anymore in modern applications. Some applications treat the mouse scroll wheel in this same manner. I suppose most people found it not very useful and applications started ignoring it, especially after the mouse became standard on computers and one could just click on the up/down arrows on the scroll bar. I would often use it if I was reading a lengthy piece of text, where I might want to scroll down as I read, but sometimes scroll back up a line or two to check something I previously read. When doing this via only a keyboard it's a handy feature to have. Page up/down would work, but it's easier to lose your place for a moment compared to seeing just a single line scrolling.
