[MiguelHudnandez]

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.

[orlp]

As long as your clothes are not to be worn in a sub-zero climate.