When you're studying, you often need to move from page to page repeatedly. You can use little pieces of paper to create quick bookmarks, but those can fall off while you work. Paperclips, on the other hand, work much better.
Redditor pooperoutlaw brings this tip, suggesting paperclips work best because they can stay on as long as you want and you can easily remove them. They're less likely to damage the page than something with adhesive too.
That said, they can cause unwanted creases in the paper or hurt the book's spine if you use too many. For quick, temporary study bookmarks, however, they do the job very well.
Use paperclips while studying [Reddit]