It's left over from earlier years use.

However it will lock the cursor keys so that you scroll the page rather than move up and down on the page.  I suppose it might be useful for too wide pages.

How mine got on I don't know, I assume I reached for something on my desk and brushed it or something fell on it and turned it on.  It was giving me fits and I couldn't find a reason in Excel for the behavior.  Then i noticed the Scroll Lock light was on and realized my mistake and felt stupid. 

I left it up in case there were others that ran into the problem.