or so, said Jacob to Richard.
"The island is a cork of a bottle keeping the evil from spreading throughout the rest of the world"
The Lost Finale (episodes 17 & 18 of season 6 aka Lost S06E17) disappointed most spectators who were highly in the expectation of answers to remaining enduring mysteries or connecting the dots. The screenwriters Carlton Cuse and Damon Lindelof chose to leave connecting the dots or filling in the blanks to the loyal Lost fans, according to me, they also gave the rest of us a choice to tolerate the contradicting metaphors or what I would call otherwise absurdities which are explained below. This could also be considered the making of a proof that they did not plan what was to come along as the story went on, neither the finale.
Jacob was there to protect the "light", an intense source of electro-magnetic energy at the "heart" of the island. The same energy - when uncontrolled - would cause passing nearby planes to crash to the island. The same energy which turned his brother into a smoke monster which is invincible, reads people's minds, kills the characters which he no longer likes or those that the island is done with, he can also disguise himself using the bodies of dead people.
The light coming from the energy field hidden in an underground chasm is supposed to be left alone never to be visited according to Jacob's late mother who thanked her rebellious adopted son for killing her. However, as explained in the Lost Season 6 Finale (Destiny Found) when Desmond Hume and Jack Shephard went there on a sacred mission to disable the unbeatable smoke monster's powers - however they didn't turn into smoke monsters themselves for entering the cave and being exposed to the electro-magnetic power of that great source of energy! For them it was just a switch-board to turn on or off the island's power, when turned off the island would start rambling and to slowly sink into the ocean.