Because it looks better and is a guide to communication.

For the poster child of a naïvely set/composed text which suffers thereby see:

https://blog.lostartpress.com/wp-content/uploads/2025/10/vir...

That a book about a perfectly made tool cabinet suffers from so many typos (the maker's name, Henry, is mis-spelled on the inside cover) and such pedestrian composition embodies much of what went wrong when the typesetting unions all bargained for sinecures rather than active participation in the Desktop Publishing Revolution.

>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 :)