We know at least partly what makes the Island special is the Prison beneath the Island. It is the Power from the entities trapped within bubbling up, that creates the leylines/leyline originating from the Island. It is possible that the Prison existed prior to the Island (within the NeverNever). Merlin then layed down a foundation on Earth of magic on the Island that created the Earthly Frame of the Prison. If the Island was located somewhere else then moved, it could be that it and it's associated connection to the NeverNever Prison was relocated to its current location.
Now to answer your question, why would this Island be important. Perhaps it is because Demonreach is associated with the Island. Whoever wanted to keep the Demonreach Island associated with the Prison had to MOVE the Island for some reason. Let's say, there was some natural calamity about to occur...Fault lines would split apart the Island, Earthquakes would place the island underwater, whatever led to the destruction of Atlantis would befell Demonreach Island. For whatever reason, Demonreach Island and it's connection to the Prison was in danger. It (Island) had to be moved and then the connection had to be reestablished. Merlin reestablished the connection and layed down the appropriate wards.