The Scroll Lock key is useless in the modern day, and some higher-end keyboards don't even have it:
Same can be said for the Pause and Break keys, but however it pauses the BIOS POST output on many computers, so it's a useful key (some older games also use it to pause the game).
The Scroll Lock key doesn't do anything: it doesn't even lock scrolling as its name suggests. I can understand that some older keyboards have this key, but not the newer ones such as the keyboards bundled with many OEM computers these days; it's not like any newer computer is likely to run an older operating system such as Windows 9x. Games that use the Scroll Lock key may not even be compatible with newer operating systems.
Is there a historical reason for this or are keyboard designers just plain lazy to get rid of this useless key?