two: you will be able to reset the map without suppresing everyone build- (it will change current location of natural build, respawn the ores and stuff like that)
and the pokemon will not spawn on the private island and if you want to make your island more huge to continue to build on it you would need to pay in game money for it-
other advantage: the players that love building will be able to build without annoying building arround that degrade the player building and without huge hole that people do in the map- it will also update the FPS in the main map- because the item in the chest will be in a personal warp that the player have to go to the island and not in the current map. (sorry for all the grammar error)