Pretty important. For one thing, the hype created around it caused such a surge in use that it grew faster than any other independent site in history. Kind of a self-fulfilling prophecy.
Of course, if it was just a hype then Pinterest would have crashed by now.., but it hasn’t and it probably won’t.
To answer a question like above you’d show a lot of statistics and just say “voilà, that is why Pinterest is important”.