Maybe the Oceanic 6 somehow make the island move forward in time, preventing Juliet, Sawyer, etc. from being killed in the purge? Just a guess... hopefully we'll find out soon.

they didnt realli need 2 come bak but the only way 2 FIX the island was 4 john 2 move it which removed him from the island and fixed it but richard wanted john wanted 2 come bak so he told john the onli way 2 fix it was 4 him 2 get them bak so they (the survivors) needed 2 relive the moments as best possible 4 the original crash 4 john 2 get bak there otherwise he wouldnt hav been able 2 get bak ther if it was just him on his own? so it was all realli about getting john bak afta he fixes the island