Why do you write on teeth using a Sharpie?

I don't write on teeth, but I do sometimes use a fine point sharpie to draw on teeth. I can try out potential modifications on edges of teeth. Often minor modifications of the edges of anterior teeth can enhance smiles. By blacking out small portions of the edge I can preview what removing that portion of the tooth would look like. I believe in the adage "measure twice cut once". The sharpie indelible ink can be easily removed with some alcohol on a Q tip, but actual changes in tooth structure are irreversible.