A few decades ago, books were often rather expensive. People didn't buy a book but borrow it from a library and they shared book with others. Or they planned to

A few decades ago, books were often rather expensive. People didn't buy a book but borrow it from a library and they shared book with others. Or they planned to sell a book later on. In such a situation, it's better to make notes on extra sheets of paper. Since books are cheap now and you hardly get anything for used books (unless they are an antiquity), it probably doesn't matter. I still think though there is a surplus value in making notes on extra sheets of papers: it's easier to skim through your notes and to find something, and it's easier to archive your notes.