It is Tolkien's adept ability at combining imagination with Sub-Creation to give his fictional world of Middle-Earth that ‘inner consistency of reality’ which points to the truth of the Gospel.
Every child builds. Some build castles out of wooden blocks handed down from an older sibling. Some construct forts out of blankets, chairs, and miscellaneous living room artifacts.
God wired us to be storytellers. God made man in his own image and that image includes a rational mind that communicates in large part through stories.