The Island keeps evil from spreading into the world.
I had to take a breath when I heard that. Sliding a little down the mystical end for me. I certainly appreciated Jacob calling it by a number of names, so no problem. Lost always tiptoes the edge. We know this.
What we don't know - well, that could go through a wormhole and fill a sideways 'verse. But, although Jacob described the Island as a cork stopping Darkness, he failed to call the Darkness by name. Which means we can't say for certain what he was talking about.
Many people have jumped on the idea Jacob means the Monster, but I'm going to suggest he meant the Infection. Dogen said once the Infection reached Sayid's heart he would cease to be "everything he was". It makes sense, if the Infection does what Dogen proposed, something would need to keep it from reaching the rest of the world. Island, cork, etc.
The Island needs to be protected, since it's so important to the survival of the world. Not an easy feat, really, so whoever put the Island in it's place finds two people to do the job: Jacob as the gatekeeper, and the Monster to keep those Jacob let through from getting out of hand. Who knows why people were allowed to come to the Island, or if they were ever meant to.
Whatever the case, Jacob and the Monster both believed in the mission at the start. But the more people came to the Island - the more they fought, destroyed, corrupted - the more our friendly Monster stopped believing. People were obviously corrupt to begin with, so it didn't matter whether the Island kept the Infection at bay. The Monster was stuck, though, unable to leave the Island because Jacob, the gatekeeper, wouldn't let him leave. For some reason the Island staying put depends on someone in the Jacob role and someone in the Monster role. Since Jacob believed people could choose between Good and Evil, he still believed keeping the Infection under wraps was important. He had no desire to see people taken over by Darkness unwillingly.
Jacob's whole point in bringing the Others to the Island was to prove the Monster wrong so he would stay. This is the reason he kept them all, including Richard, in the dark about the truth of the Island. Yes, he told Richard about the Darkness, but he probably left it to Richard to decide for himself what it was. Dogen called the Monster Evil Incarnate, which leads me to believe he thought the Monster was the source of the Infection. But Jacob never intended the people he brought to the Island to protect it, until he started looking for Candidates.
Now that Jacob is dead, the Island is teetering. Up for grabs. The Infection has already filled the Spring in the Temple, a place we've been told used to heal at one time. If a Candidate doesn't take Jacob's place before the Monster leaves, the Island may be unable to stop the Infection from spreading to the rest of the world.
So this didn't start as a struggle. Jacob and the Monster were on the same side at one point. But when the Monster lost his faith and Jacob wouldn't let him go, he decided the friendship was over - and he would leave the Island at any cost.