[abdullahkhalids]

They are useful for someone in a nearby field trying to learn this field. That person first reads the textbook, and a few specific papers. Then once, they have a good narrow understanding, they broaden it by reading one of these review papers.

In essence, a review paper saves you the trouble of doing a literature review in a new subfield, because it identifies the important papers for you.

That said, the reason review papers are usually written is for the authors to cement their own understanding of the network of research in the field.