Notebook margins were invented because of...

Originally, margins were not made to leave space for outraged teachers' notes.

The truth is that in the past, books were often victims of mice and rats who liked to eat away at the paper. In order not to let the rodents eat pieces of information with the paper, people started to leave empty spaces around the edges, where the damage was most likely to occur.

In simple to save the written information long run to avoid the damage of books from the mice or rat people used to leave space that is margins because most of the damage occurs on mice bites throughout the edges only. So people used to leave margins space to protect the written information.