In the end, the island was a place where balance between good and evil was put on a scale and people were tested to see how they measured up.
(If anyone wonders why Micheal wasn't in the church in the final scene, it was because, as he said in his last scene a few weeks ago, he couldn't move on. He had failed the test and was doomed to roam the island for eternity.)
The center of the island was a place where the light of all the souls of humanity was shining just above the fires of hell. That's why there was a need for the cork. The cork wasn't meant to keep MIB on the island but rather to contain the fires of hell from spreading.
When he was thrown into the pit, MIB was convinced that the only way to get off the island was to return to the cave and put out the light. Without knowing it, he was being used. Or maybe that was his pact with the devil: If the light was put out, the devil would give MIB's life and humanity back.
Jacob, with almost infinite patience, figured that the best way to prevent MIB from putting out the light was to hide the cave. (Jacob knew how to use mirrors!) Over time he did put in place a back-up plan in case MIB found a loophole. Jacob's back-up plan was Desmond. Jack and Locke both figured that out and, while Jack had faith in Jacob, Locke thought it was his opportunity to finally destroy the island and leave.
What Locke didn't know was, when Desmond took out the cork, Smokey wasn't needed anymore so Locke lost his powers (or the devil honored his pact at the most inopportune time). The devil had been freed, the destruction of the island and the world was underway. Only Jack, as protector of the island, could put the cork back in place and save the world.
That's for the fantasy story behind the island as I perceive it. The story of Lost however was the redemption of the characters, how their love for each other enabled them to overcome and how it helped them find themselves. They needed the island for that just as much as the island needed them.