The more I think about it the more I think things are coming a little clearier, the fake 815 crash was stage by Ben's off island group. Because Ben knows that Widmore is looking for the island for what reason I'm not too sure about.
When the original 815 crash happen Ben sent someone to each part of the broken up plane to make a list of who was on the plane and who were the surivours. Ben check this list to see if any of them might be of Widmore group. Ben thinking that Widmore had found the island. So Ben get his list and sees no names jumping out at him that could be Widmore group. So then Ben's off island group stage the fake 815 crash to throw Widmore off the possibility of finding the island once a search has began for it's black box.
It was pure accident of how 815 had crashed at the island because Desmond wasn't where he was suppose to be to enter the numbers when he suppose to which cause the magneto thing to put the airplane out of the sky.
Now the rest of this is just me thinking okay. Jacob is having some kind of fallout with Ben and thinks it time to replace Ben, so Jacob is search out the surviours to see whom might be the best choice to take over Ben. Jacob has a plan for each one of the surviours and onces they've fulfill that use then the island's done with them. Once everything is in harmony with Jacob all it good once again. I know it's a little crazy, haha.

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Standing in the ocean with the sun burning low in the west
Like a fire in the cavernous darkness at the heart of the beast
With my beliefs and possessions, stopped at the frontier in my chest
At the edge of my country, my back to the sea, looking east