Em-dashes add a bit of a pause. And having them longer and taking a bit more of horizontal space makes it more intuitive. They also break a sentence into parts. Having them easily distinguishable helps navigate text and reduces overhead. Just like periods or paragraph breaks help you see parts of a text, or syntax highlighting helps you see lexemes in a program.

Using just one dash for everything will be readable in a text message or comment. But not in a (complicated) book, because there the benefit of these small things gets multiplied by the scale of the book.