Apparently we say "eh" because it's polite! "If I say “you’re an idiot, eh?”, what I’m saying is, you’re an idiot, but you should also think you’re an idiot, and our understanding of you as an idiot finds us on common ground."
...and if you liked that one, here's one of my favorites from a few years ago, on why Canadians like to add a certain phrase to the end of so many sentences, eh?
"Eh?" Definitely not as ubiquitous throughout Canada as the stereotype of CDNs goes (regional)...