Rats were protected by the margins
Most people believe that notebook margins are for teachers to write notes in, however, this is a widespread fallacy. In actuality, margins began to be added to papers in order to safeguard people’s labor.
Earlier in history, it was typical for rats or insects to nibble off the margins of your paper, thus anyone who wrote all the way to the edge would lose work. Large margins provided the rats more area to eat while reducing the risk of them damaging a piece of paper with writing on it.