Why? Because there is only upside and no downside to having 3x of this card in your deck, as long as you keep meeting the requirement of a unique Silvan and Dwarf trait character. And in early game with the aforementioned heroes, meeting the requirements is a sure thing.
Let's look at the two upsides:
(1) The card has zero cost and draws 1 card, shrinking your deck as you use/discard this for free and replace with the next card. This makes your deck effectively 3 cards smaller (47 cards instead of minimum 50), so all your other cards have a greater % chance of being drawn into your hand.
(2) The above by itself would make the card worth including x3 - but then it also adds one resource to any hero you control!
Bonus Combo: Add a readying benefit (and choose the exact timing of it) by adding that resource on a Heir of Mardil Hero.
And if you have a "perfect" 50 card deck already? Add 3x of this card anyways (if you meet the Silvan/Dwarf requirement) - it makes your deck effectively the same 50 card size, but you still gain the benefit of additional resource generation.
Multi purpose, interesting combos, free to play... this card is magnificent.