If there's one thing I learned after watching Lost, it's this: there is no such thing as a pointless detail. It could be an Easter egg, it could be something that characterizes an important element of the show, or it could be a tool to help someone, either the characters or us watching, understand what the heck is going on.
Like, say the flash sideways. There surely is a point in showing what happened to the most important characters of the show after Oceanic 815 successfully lands in Los Angeles. Sure, they're totally different from the get-go: Jack lost his father's body, Locke lost his knives, Kate lost her captors, Hurley lost his bad luck, Sayid lost... well, he didn't really lose anything in the alternate timeline.
But it's fun seeing the parallels between the two storylines. Say, Kate escaping Edward Mars. During the crash, Edward was knocked out after some luggage fell on his head, enabling her to grab his keys and escape. In the new timeline, he was knocked out after Kate banged his head on the sink. Look at his head wound. Pretty much the same place, right?
After the crash, Jack became the leader, helping his fellow survivors deal with the immediate aftermath. In the new timeline, Jack became the go-to guy, removing that bag of heroin from Charlie's throat, saving him just in time. Locke still lied about his paralysis, much like what he did on the Island after the crash. (Also helped that he chatted with Boone about stuff, echoing their fatal closeness.) Sayid and Hurley, well, their lives seemed to go just fine--could be because of their supposedly special qualities, or because something's yet to happen to them.
Now, think about a few seasons back, about the conversation between Eloise and Desmond back on "Flashes Before Your Eyes". You cannot change fate. What's meant to happen will happen. You don't stop things from happening; rather, you just delay it.
Now, think about the conversation between fake Locke and Ben, after Bram was killed. The thing that made the actual (and now really dead) Locke different from the rest of the survivors, the former said, was his desire not to go home, recognizing that his life outside the Island was, for lack of a better term, pathetic. It's what the rest don't recognize.
What if the flash sideways was a tool, for the survivors on the Island to realize that there's no point in leaving the Island? Jacob touched Jack and Kate and Locke and Sayid and Hurley and Jin and Sun for a reason. Perhaps he found something special in them. Perhaps that's why he wants them so badly on the Island. Perhaps he knows that, one day, the Man in Black will get to him and kill him, destroying the balance of the universe. Perhaps he wants the survivors to be part of the dynamic, much like Cindy and Zack and Emma are now.
Perhaps the survivors still want to go home, some more than others. They'll be shown what happens when the plane does land: things may look just fine, but they'll still end up where they are right now. Jack will face troubles. Locke will be healed. (Jack will fix Locke? Sounds plausible.) Kate will still be arrested. Jin and Sun will still face marital troubles. Sawyer will fall in love with Juliet but they'll be forced apart soon. Their fates are inevitable, just like Eloise said. And then they'll realize that they have a much bigger purpose, one involving staying on the Island for good.
Now, that may sound fantastic. Why show us the flashes now if they aren't real in the first place? One more thing I learned after watching Lost: wait for things to pay off. Someday these two timelines will make sense--and it will be for something much bigger.