I love them because it is so easy to mark my quilts! I can take an iron to the marks and "Poof" ~ the marks are gone. If i make a mistake marking my quilt, I can take an iron to it, the marks disappear, and i can start again. I do wash my quilts before I give them away, and no one i know will refreeze the quilt.
In today's world, we use all kind of marking utensils that "disappear" w/ even just the air that we don't know if they're really "gone" or they just appear to be gone.