A little late on this since I just found this site, sorry. But it was written immediately after the episode.
Here is my guess after the first episode of Season 5, which is completely different than my guess after Season 1, which could be completely different after seeing the next episode.....Locke is Jacob and he is hanging in time until he gets reset. The easiest way for that to happen is for Locke to give Alpert the compass at the very beginning of the story when Alpert comes to his house as a child (give me the compass next time you see me). If that would happen, everything else would, kind of, fall into place. This means that Locke would come to the island early, learn about the island and take the lead when he's supposed to instead of Ben. Locke not giving Alpert the compass tells Alpert that the timeline they are on at that time is not the correct one (more below) so he leaves.
Since that probably doesn't happen, the next way to reset everything is for the island to find it's 'constant', the Oceanic Six. Once they show up, the island settles (dead become dead, alive become alive) and time moves on from that point. Depending on when/where they actually show up on the island will determine how far it is after the 815 crash.
Ben isn't supposed to go back to the island and that's where he becomes a bad guy this season. His time has passed and now Locke is in charge. Bringing Ben back is going to cause more problems as he has no reason to be there (no daughter, no followers, etc.). Unfortunately, he's one of only three people who know how to get to the island, two now that Ben had Locke/Bentham killed/iced/comatized. When Ben reaches the island with the Oceanic 6 he's going to try to take it over again with them helping him. He needs Locke to get back to the island to 'set' it but he can't afford him to be able to talk to tell the rest of the Oceanic Six what Ben's plan is.
Why do they have to go back to the island and reset it? Because if the island keeps time hopping it's going to continue to become unstable. When I say 'it' I mean the tremendous amount of energy within the island. It's just going to continue to boil until it either settles or boils over (God help us all) and destroys everything. The island needs to be reset but at what time it will be reset is to be determined. I think there are three choices:
- Early Locke when he was a boy - When Alpert gets the compass from him - Oceanic 815 crash time - this is what I think it ends up as - The current time, three years after they left the island - Ben tries to take over the island again
The time travel theory that they haven't talked about that I look forward to, or am hopeful about, is the multiple "time lines" that happen during the time travel. For example, Line A is the reality that we just saw over the first four seasons but if they go back in time and change something then Line B splits off at that point. Line A and B still run, just in different dimensions....dimensions that they may be able to talk between (whispers in the jungle). There is a scientific name for this but it's in a book I have at home.
Some of the other character plots and plot lines (Desmond and Penny) (Widmore and Ben) (different dimensions) are interesting and lengthy so I'll forgo that stuff at this time.