Supposedly, what the scroll lock key would have done in the old days was this:
When the computer is typing out to a screen in DOS text mode, when it has to type a new line after the last line of text on the screen, instead of going to the trouble of moving the whole screen up one line, it would just blank the whole screen, and start putting the new text on the top of the screen.
Instead of quite doing that, it might instead invoke an intermediate mode where perhaps the last 8 lines of text on the screen are copied to the top, and the new text goes to line 9.
This was to reduce the overhead of displaying text on the screen, which is why the functionality quickly became obsolete as computers became more powerful, but I've worked with old terminals that had a similar function.