In chapter 19 of Frankenstein, Victor says
I traversed the northern highlands, and fixed on one of the remotest of the Orkneys as the scene.
The Orkneys are made of an archipielago (a chain of islands) located off the Northern part of Scotland. They are fertile, as opposed to what Victor said about them being barren. They are also considered "desolate" yet, out of a total of approximately 70 islands altogether. there are around 20 islands that have been consistently populated.
The seclusion of this location, and the physical distance that Victor is willing to travel for the sake of his project, may be part of the significance of using the Orkneys as the place for the creation of the female monster. However, another possibility for the importance of this specific place is that it has a history that dates back to the earliest Europe, all the way to the New Stone Age Era and the advent of the agricultural revolution. The remains of the earliest civilization of this pre-historical time is found in the Orkneys in the form of the village of Skara Brae. This well-known fact is perhaps one of the many points of interest that led Mary Shelley to use this location: a place that opens the door to our past. Should Victor let his own past repeat itself, and create a second creature?
Regardless, Victor's mention of being at the farthest of the islands has a flaw in his calculation, because he claims that he is at a distance of five miles from the mainland. Depending on whether he means to say that he is 5 miles from the biggest Orkney (which is also known that way, "the mainland"), or 5 miles from Scotland (which is impossible) we could place more weight on the use of the Orkneys as the intended birthplace of the female creature--which never got to be created.