[jmkb]

It's an early implementation of the same idea as the mouse scroll wheel. Engage scroll lock, and the arrow keys are used for scrolling the current text view instead of moving the cursor.

The idea of also using it to pause scrolling console text, I think, originated in Linux.

[bitwize]

There was a PC Magazine utility from the 80s, WAITASEC.COM, that allowed you to use Scroll Lock to page through command output that had scrolled off the screen.