>Because it looks better and is a guide to communication.

Personally, I think em-dashes (when in a font that shows the damn difference) look exaggerated and silly. They certainly don't provide any important guidance to its meaning that I can't derive from context.

That was a cool toolbox BTW :)

Some typographers agree and afford a 3/4s length em-dash.

Yes, the Studley tool cabinet is amazing, which makes the infelicities of the book all the more enraging.