The Flash-Sideways Are Important

Since "LA X" I've been trying to figure why the flash-sideways are important.

The characters' lives have been significantly altered: Jack has a son, Sayid is not with Nadia, Sawyer is a cop, Locke and Helen are getting married, etc. Their story-lines still intertwine, but its hard to guess where the alternate reality could be heading.

Some see it as an epilogue for the characters' story arcs. In theory the Losties may sink the island in the original timeline in an attempt to change their destinies. The result would be the Flash-Sideways. But as we can see, by sinking the Island along with Jacob, the affects it has on their lives extends to before the crash.

The problem with this theory means the Losties would need to travel back in time far enough to rid Jacob from their lives' entirely, something not to far fetched for LOST, but I don't think time travel is the way it is heading this season.

Also, I don't think the flash-sideways would be a satisfying ending. The characters go on living their lives as if they have never met and the events never transpired. It would make the past five seasons a complete waste of time.

What I believe the flash-sideways are is the exact opposite, it's a prologue.

In the flash-sideways the candidates are still unknown to Jacob. They live their lives making their own decisions.

Now, if you remember back to the episode "Sundown," Dogen explains to Sayid the importance of his baseball. It serves as a reminder of the mistake Dogen made in his "previous life" and how Jacob offered to help him correct it. Dogen's previous life is seen in Jack's flash-sideways episode "Lighthouse." Dogen is with his son, who will eventually be killed because Dogen made the mistake of drinking and driving (ironic). Jacob visits Dogen in the hospital and offers to save his son if he comes to the island. For this to happen, Dogen must relive his life influenced by Jacob, which is the timeline we have seen over the past seasons. Dogen may not be his son's father in the original timeline, but his son does exist. Dogen is eventually brought to the island where he is shown his past life and accepts his choice to change his fate.

Eventually, in the flash-sideways we will see the rest of the candidates make their own mistakes, only to be visited by Jacob to offer his help in exchange for coming to the island. They will be unaware of the promise they had made in the the next life(the original timeline) and will not understand until they are shown.

This also explains how they came to be candidates in the first place. Jacob didn't chose their destiny, they chose to go to the island. Once Jacob knows they are willinging to change their lives he sends them to the alternate reality where he monitor's their lives from the Lighthouse, intervening with the decisions that will push them to the Island.