In addition to the above uses, one I've discovered is that when I have a starched or 'sized' fabric (Magic Sizing, which I use alot), any marks made on my top wash out so much better - regardless of the marking tool used. Please note I only use normal quilt marking tools, such as blue washout markers, Crayola Washable Markers, pencils, etc. But they do seem to wash out easily and I've never had one stay in permanently. This could be just luck, the fact that I would NEVER iron a fabric with a mark on it...or it could be the layer of starch/sizing I put heavily on my fabrics washes out easily and the marks on it also wash out. I don't know...just know it's so for me ;)
I do a lot of marking, in piecing maybe not so much, but certainly in longarming.
Debbie in Austin