|Arda - The World|
Tolkien referred to this urge to create a fictional world as "sub-creation" i.e. that one was expanding upon the Creation that God formed. For Tolkien, it was vital, if the sub-creation were to be "true" that it be congruent with the world as we know it, and that it not conflict with the Biblical account of the world, and the arrival of Christ. Hence, Middle-Earth is our world in a distant past inhabited by virtuous pagans, who fight for the good, whilst being ignorant of the Resurrection (well, it hadn't happened yet....) OK, for me (as an agnostic) that isn't really a factor, but I do believe that the creation of fictional worlds is a valid past-time.
Anyway, I seem to have veered into my undergrad dissertation, so I shall stop there!