Plays an important role in B12 formation, which plays a key role in energy metabolism. Cobalt is really important for the rumen microbes to synthesise vitamin B12. Cobalt is also believed to play a role in certain enzyme reactions.
While it can be hard to get exact inclusion rates 0.1mg/kg has been quoted. Sheep are more prone to cobalt deficiency, particularly young growing lambs. This can be attributed in part to it being involved in wool growth.
It is key to understand that vitamin B12 plays the key roll in energy metabolism. This is important in the production of glucose. This is important around calving time for the cow around energy metabolism. It usually is poorly retained within the animals so needs a regular supply at key times.