This is not a question that can be answered briefly!
I have written several posts since January 2013 on the coil rod and its use in both circular and single-plane warps. It is one of a few ways ways I have found to solve the problem of corrugation in warp-faced plain weave when using cotton as it ensures even tension across the warp at the weaving line. It can also help maintain width and keeps the threads beyond the shed rod orderly.
Start reading about the coil rod in my first post The Curious Coil Rod.
Here is Burmese weaver, Dar Ku, with a coil rod installed on her backstrap loom. It is the furthest stick to the left.