Symbolically, if the island is representing some deeply revered god then it becomes a symbolic bridge to that god - the means by which one passes from the physical world of the mainland to the deeply spiritual world of that god.  Thus if you walk a causeway, for example, to a sacred island you are in some respects performing a sacred act and one which in the olden days was taken extremely seriously.

In all the Mysteries, there was a part of the ceremony that involved the traversal of water to get to the 'island' that contained the buildings used to perform the Mystery ceremonies.  Very often this causeway was guarded by Guardians emphasising the importance of the whole occasion.  In Ancient Egypt, the Colossi of Memnon served as guardians to the complex that was at one time the pyramids and the labyrinth.

When completed in 1893, the creation of the Corinth Canal turned Corinth into an island ...