What are the reins used for?

Reins can be attached to either side of a bit or head collar. Head collars are mainly used on Camels & Oxen.
Whether the reins are attached to a bit or straight onto a head collar they achieve the same or similar result they are an extension of you arms and are able to pass signals to the animal, these can be, go to the Left, Right or stop.
A bit is generally mettle in olden days it could be wood, modern bits can be Vulcanite, I see no reason why they might not be made from other hard wearing materiel such as any of the hard plastics.
The Bit goes into the mouth of some animals, mostly equine, but have been used in camels, and possibly Oxen.
To Sum up reins are used to steer an animal.