Cobalt is added to agricultural fertilizers in many  parts of the world to counteract deficiency of this mineral. New Zealand soils are deficient in Cobalt, as well as some other important minerals, such as zinc: this is why cobalt  is a common component of New Zealand agriculture fertilizer.
Cobalt is a component of Vitamin B12. If you are deficient in Cobalt then your body might not be making enough Vitamin B12 in your intestines.

Lack of Cobalt can cause fatigue and impair the healthy growth of children. Children’s growth may be stunted as a result of lack of cobalt/B12 in the diet.

Cobalt is necessary for the development of healthy red blood cells: plenty of red blood cells helps protect  your body against cancers and other disease. Insufficient cobalt/B12 in the diet may cause a condition which mimicks anaemia.

What happens when you get too much?
Excess Cobalt can cause imbalances of other minerals and vitamins.:
Excess cobalt hinders iodine absorption, which begins a chain reaction: Insufficient iodine, in turn,  will upset the thyroid gland, causing it to become swollen as in a goitre condition.

Insufficient iodine leads, then, to increased cholesterol levels, and this in turn affects the heart.
So an excess of Cobalt really is a bad thing.

The remedy for removing excess Cobalt from the system is to have plentiful protein in the diet. Adequate protein in the diet will protect your
body from absorbing too much Cobalt. Ideally, you will eat fish at least once a week to supplement both your iodine intake, and your supply of protein. This will ensure that your cobalt absorption is properly regulated.