In some cases refrigerating the ink will make it visible again, but not reliably. Leaving notes in your car can cause then to be erased. So these are a novelty

In some cases refrigerating the ink will make it visible again, but not reliably. Leaving notes in your car can cause then to be erased. So these are a novelty or for casual use. A fantastic use case though is in sewing. Ironing the fabric makes the ink transparent. These pens are far superior to chalk for that reason.