What is so special about Glubbdubdrib island and tribe that governs it?

From the text:

Glubbdubdrib, as nearly as I can interpret the word, signifies the island of sorcerers or magicians. It is about one third as large as the Isle of Wight, and extremely fruitful: it is governed by the head of a certain tribe, who are all magicians. This tribe marries only among each other, and the eldest in succession is prince or governor.