I do not think it is reasonable to say that it is just for aesthetics, however its purpose is not clear.
We know that there is a special hidden beach in the back of the island. It seems to be only accessible once the user unlocks terraforming. In this case, that beach is on the same side of the island as the second island. It is potentially possible that the second island is an unlockable related to this hidden beach. I wouldn't put it past the game to allow for such an extensive "post game" on a game like this. It's not like adding a second island would be hard to pull off from a development standpoint, and you know the game's fans would love it.