What is the point of beating Heroic mode for any adventure?

So I recently bought the League of Explorers as one of my first paid items in Hearthstone. I loved it, and how interesting all the cards were.

Now after I beat the default bosses, a new mode showed up named 'Heroic'. When reading the description the only thing different from normal mode is "Heroic mode is NOT for the faint of heart!", and within the section itself there was no treasure chest icons like in Normal.

What is the meaning of doing Heroic mode? It seems beating it should give you the golden version of the card normally unlocked but there is no text describing what you get.

The Adventures are usually very easy to not scare away casuals. To give experienced players a challenge as well they made the heroic mode. You fight the same bosses there as in the regular adventure, except they are a lot harder to beat.
While you can usually take your ladder decks to beat the normal adventure, you really need to fine-tune your decks for the heroic bosses.

To answer your question, the point of heroic is to offer experienced players a challenge. The only prize you get for beating all heroic bosses is a special card back.

It seems beating it should give you the golden version of the card normally unlocked [...]

Fun fact: Blizzard was actually thinking of doing that but they ended up not to, because they did not want to have too many things in the game, that casuals can not achieve.