Desmond is Jacob's loophole by ivy_driveshaft

Consider why Widmore tells Desmond he is important. He is the only person in the world to have experienced an enormous electromagnetic event and survive. That event was turning the failsafe key and detonating Jughead.

Think about what that means. In order for Desmond to set off the failsafe, he had to be brought to the Island in the first place and then to the Hatch.

Desmond tells Locke that on the night Locke bangs on the hatch door until the light comes on, he was about to kill himself. Locke prevented Desmond from killing himself, which would have put a halt to the plan.

[FLASHBACK]

[We see Desmond in the Hatch, holding a gun. He's finishes off a bottle of booze, and opens the Dickens book. A letter slips out. We hear a VO of Penelope reading the letter.]

[Desmond sobs and throws the bottle.]

DESMOND: It's all gone. It's all gone!!

[He starts pulling all the books out of the bookshelf and throwing them. Suddenly, he hears banging and hears something which turns out to be Locke from the night Boone died.]

LOCKE: [from outside] I've done everything you wanted me to do? Why did you do this? Why?

[Desmond turns the light on in the Hatch. He laughs.]

[Back in the Hatch Desmond opens his Dickens book and finds the failsafe key. We see the timer at 29 seconds. Desmond rushes to open the grate and get to the failsafe.]

DESMOND: 3 days before you came down here, before we met, I heard a banging on the Hatch door, shouting. But it was you, John, wasn't it? You said there isn't any purpose—there's no such thing as fate. But you saved my life, brother, so that I could save yours.

Later, Locke is the one who becomes convinced that they have to stop pushing the button. He destroys the computer, and as a result Desmond has to go use the failsafe key.

It seems to me that Locke was pretty important in creating Desmond's "special" circumstances. Without his intervention in the hatch, Desmond would have killed himself.

Do you remember how Locke first found the Hatch?

[Shot of Boone and Locke in the dark hiking up a rocky area.]

LOCKE: No, Boone, we're not lost.

BOONE: Sorry, it's just... I don't see how you can still be following this trail. I think we should go back.

LOCKE: Don't you feel it?

BOONE: Feel what?

LOCKE: It.

BOONE: Alright, John. I'm going to follow the strips back.

LOCKE: Suit yourself. [Boone looks flummoxed.] Boone, you need this more than I do.

[Locke tosses Boone a flashlight. It lands on the ground but makes a metal k-thunk sound. Boone kneels down and grabs the flashlight.]

BOONE: What is that?

[Locke starts digging plant material out of the way, exposing a piece of metal. Locke bangs it with his knife.]

LOCKE: Steel.

It was like he was guided there, but not by a spirit, like Christian leading Jack. He just had a feeling. I think that Locke was doing what Jacob wanted him to by saving his life and making him turn the failsafe key.