What is the red line on paper for?

Curious

It's called the margin line. You know how people are always writing to the edge of the paper, and then writing really small to try to finish the word, so all the letters are small and cramped? Well by having the margin line there, you try to end the line there, and that way you don't run off the paper. Also it gives you a place to hold the paper without covering up some of the text.