In my opinion, basically that it wasn't just the same top sides. That a championship side in the cup could go far, it gave life to the cup that was falling off at that time (I know it's got worse since). If I remember right it boasted tv viewers as the classic underdog story. And gave a wider appeal that there was other French teams other than Catalans.
Now obviously it hasn't panned out since, but we also have to remember. The sport was in a different place back than, and in my opinion not a great place.