To me the island is the keeper of Time. It's Time Island. It's important to protect the island because without time we'd all be dead. The writers hint and hint at this in all 6 seasons. I get this idea from episodes like "Flashes Before Your Eyes", "Not in Portland", -from the name of the Labs Juliet is to be employed, 'Mittelos', which is an anagram for 'Lost Time'. Also, in season 6 Jacob's fake mother says everyone has a little bit of the light inside them and it's the one thing that everyone wants più of, of course, she's referring to time. If time had a material form it would probably look like light. Lost is all about the island needing a guardian (the candidate's purpose) to keep it alive.