Dark Link is usually created for some purpose.
In Adventure of Link, he was created to test if Link was truly worthy to claim the Triforce of Courage and awaken the sleeping Zelda, so he wasn't so much a villain as he was a test.
In Ocarina of Time you could argue Ganondorf created him after watching him fight in the Forest Temple and made a creature with those fighting skills in mind.
In Oracle of Ages, several Dark Links appeared as foot soldiers during the final battle with Veran.
In Four Swords it isn't explained where he game from so you could argue Vaati made him, though canonicity is questionable.
In Four Swords Adventures he was created via a dark mirror that no doubt made him through Link's reflection.
In anything else, he's just a representation of someone else or a bad guy in a mini-game.
So there is, in fact, not one Dark Link. There are multiple, each made for different reasons, mostly to be an adversary for Link.