I asked this in an /r/politics thread talking about the DNC Chairwoman. Who said that "Super-Delegates exist to protect party leaders from grassroots competition".

Someone attempted to reword the phrase as "Superdelegates are there to avoid a repeat of 1968 and a disastrous convention. Yes, they've been asked about their preferences now, but when the time comes they'll go with whoever has the popular mandate." (/u/johnnynulty).

So what happened in 1968 that made Super-Delegates necessary?