I think that adage came about from the old days when it was not uncommon for users to come into contact withe live parts of metal. Wet vs dry hands made the difference between a tingle vs a severe shock.
I used to moonlight at a company that had a room full of expensive test equipment. They had several fire extinguishers 2 of which were filled with what they told me were putifed water that did not conduct electricity. I could only image how much damage they could do when water got into test equipment.