Apart from helping you find important passages in a book later on, sticky note bookmarks also allow you to add additional context to the section you highlighted (e.g. why you bookmarked a particular section or thoughts you had about it).
You could write down notes like this in a separate notebook, but then you’d lose the connection to the source they are based on. What makes post-it notes so interesting is the spatial relationship between the notes and their respective context.
It’s this spatial relationship that also make post-it notes great reminders.
We really don't have anything equivalent to sticky notes' versatility in its ability to be placed in context of the ideas or objects it annotates. It almost seems like the way software is built these days makes most software impervious to this kind of rich annotation. I wonder how we can break this constraint?