A “mana dork” in Magic: The Gathering is quite a silly term. Basically, it refers to creatures who can provide you with additional mana in some fashion.
Cards like Elvish Mystic can really spruce up your strategy in the right deck.
In Magic, mana is of course a valuable resource. With mana, you may play the cards in your hand and pay for other expenses within the game’s mechanics.
Using cards like Elvish Mystic to provide you with the extra , or another color for that matter, can lead to big advantages for your match. You can have extra mana to play more cards. You could use the additional mana to “ramp” toward expensive spells faster, such as Craterhoof Behemoth.
“Mana dorks” are certainly useful, but they are not meant for every deck strategy. I utilize a full set of Elvish Mystic cards in my “green ramp” and elf deck, but I would not use them without a purpose.
This is because “mana dorks” are generally poor in combat and are vulnerable to creature removal.
Still, cards like Elvish Mystic should be appreciated because they can be incredibly potent when used to their full potential.