Line numbers of course were critically important in the punch card days.  Card columns 73-80 were reserved for line sequence numbers.  Some of the fancier keypunches could read a card deck and put the line sequence numbers in for you.  A sequenced deck was a godsend if you happened to drop the deck on the floor.  Just put the deck in a card sorter and sort on cols. 73-80 and you're back in business.

In editors for non-screen terminals the line numbers provided an easy way to reposition the editor within the text stream.

-Paul W.