ben tells richard that he comes from the future. daniel tells ellie, i'm sure this will happen over and over. perhaps the 6 have to go back because plans were made (who knows what!) based on having the people from the crash there. they didn't seem surprised that the plane crashed there, they had spies and got names of who was there. they knew so much about each passenger (and that's still a mystery of sorts, how they got that info so fast) and didn't seem all that surprised. maybe they were waiting for the crash, based on stories handed down from the 50s, that people would come from the future, that someone would save them (from what?), that kind of thing? so maybe they only need the 6 because they were foretold they would be there?