The Temple of Isis originally stood on the island of Philae near Aswan. Once the old Aswan Dam had been built, Philae lay under water for 9 months of every year. The development of the High Dam would have submerged the temple year-round, and so the decision was made to relocate the temple to the island of Agilkia.
Traditionally Egyptian temples are built on an axis with a straight line running between the front entrance and the altar room. The island of Philae was too small for the temple of Isis to be built on an axis and some initially thought that the Egyptians made an error in judgement in choosing the temple site. It is hard to believe that the same culture that was able to mysteriously build the temples of Abu Simbel with such exactitude would make this mistake.
In actual fact, Philae was chosen for its mythological significance. Isis was the devoted wife of the God Osiris. Osiris’ evil brother Seth killed him, dismembered him, and hid the 13 body parts around the kingdom. Isis managed to find 12 of the 13 body parts and used her magic to piece him back together. Philae island is the place where Osiris’ left foot was buried.