it's a throw back to early days on computers, it main purpose is to either stop text scrolling on the screen, or modify the way the arrow keys act.
Try the arrow key one for yourself, open an excel spreadsheet and you can move about the individual cells using the arrow keys, now turn on scroll lock and see what the arrow keys do.
But by and large it is a redundant key now and not used for anything
(oh and if you feel like playing silly ******* you can enable a registry setting in windows called CrashOnCtrlScroll and then if you hold down CTRL and double hit the scroll lock key it will crash the system, I assume this is in there for debugging reasons :))