Imagine a crowded shelf (or just look behind you). When binders and books are tightly wedged onto a shelf it can be hard to get them down. A finger pull, with or without metal, allows an easier way to get what you need.
Sometimes these are called European finger pulls, as they are more common in Europe than in the United States. Adding a metal-less finger pull allows the same convenience of access and a much more easily recyclable binder.