In the season 4 episode "The Constant", Faraday actually had already written in his journal that Desmond was Faraday's constant. I believe the exact writing was, "if anything goes wrong, Desmond Hume is my constant". And when Faraday is talking to Desmond, he mentions that Desmond is special that the rules don't apply to him. I can only surmise that the rules of time travel "What happened, happened" etc.. dont' apply to Desmond.
Desmond does seem to be special in regards to time travel and the idea of predestination and has been involved in several time travel paradoxes.. Remember he had already met Faraday's mother (Eloise) during his flash after turning the failsafe key in the hatch. He had traveled back in time
Upon returning to the island, he awoke with a similar ability to Eloise - in that she (and he) obtained some small ability to see the future. Desmond was able to see the future of Charlie and, to a small extent, put off the change but was not able to completely change the course of the future ("What will happen, will happen").
And in the constant, his mind went back to the past (1990's I believe) before finding his constant in Penny..
So, as the island travels through time, the people on the island need their constant - perhaps that is why they needed the oceanic 6 to return - or maybe the island itself needed them back as a constant to stop jumping in time..