1. They are cheaper.
2. They are good for the environment. By buying them you save paper, hence trees, that would have gone into the new book.
3. They have the important sections marked.
4. They are already creased so it is easier to open them
5. They are already creased so you don’t have to worry that you’ll crease them.
6. They connect you to a person. Sometimes there are funny comments at places by one of the previous owners.
7. They offer better return on investment. Their resale value is better as compared to the resale value of a new book.
8. They have a certain surprise element. Chance discoveries like an old dried flower or an interesting bookmark are real possibilities.
9. By buying them you give use to an item that would have remained in the owner's store room for years and years.