Indexing rings have many uses. They are attatched to a lathe headstock and while the lathe is switched off ( and preferably unplugged ) , a pin locks the ring in a chosen fixed position. This allows holes to be drilled, slots to be cut or routered etc etc.
Basic rings have 24 holes which would allow even spacing of 2,3,4,6 holes or other cuts.
Some rings get very complex and their uses is only limited by our imagination.