You can make security ink. Dissolve some in water. You get a pink colour, then use a feather to write on a pink blotting paper. If it's dried the writing is not visible. Hold the paper in distance above a candle or use hairdryer. The colour change to blue. And the writing is visible.
Other purpose is to indicate wet areas or air. The heated cobaltchlorid )ooses water and is blue. If its get wer it's pink again.
Beware Cobalt compounds are poison.