You have a certain amount of provisions, which you have to make last until you find a way to make the island sustain life—or convince someone to send you more.
You don't know how big the island is at first or what predators lie in wait.
There's always a chance someone else will raid your island if it looks fruitful, so you need to shore up your defenses.
Eventually, if you're successful, you'll be king of your own prosperous world. If not, you'll die—or, at least, have to go home.
Either way, it's a fun adventure (until you get eaten by a tiger).