So the Oceanic 6 had to recreate as much of their original flight as possible in order to get back to the island. The way they got to the island the second time was by plane, just as with the first time. If they know where the Island WILL be at a certain point in time, why not just take a boat out and wait for it to appear? But the plane is important.
In early seasons, I think it was Locke who asked Jack what he thought about them surviving a crash that should have killed them all. Or someone at least mentioned this. I think that Oceanic 815 was filled with people who were meant to be on the Island . . . by people working with Linus, Widmore, Hawking, etc. Jack got onto the plane because Christian died at a convenient time. Claire got on because the psychic bought her a plane ticket. He selected the very flight. Sun and Jin got on a plane arranged for them because of Jin's work. Sayid was also given a plane ticket. He took the next day's flight, but it's possible that if the people who gave him the ticket were working with Ben et al, they would have known he was going to bury his friend, to manipulate him into going on the next flight. Or if Sayid had taken the ticket they would have said, "Oh crap, wait, this ticket's void. Here, take this September 22 ticket." Kate's flight was chosen for her, but by the marshall, who probably wasn't in on it considering he was after Kate because she was a criminal . . . unless he was so engaged in the hunt because he wanted to make sure that HE was the one to capture her and get her onto the plane.