Tolkien is of course best known for writing The Lord of the Rings novels, as well as their prequel The Hobbit, both of which have been adapted into widely popular films. Both of these series take place entirely in Middle-earth, a fictional continent in the center of the earth about 6,000 years ago. While Middle-earth is technically only a setting for Tolkien's major works, it is colloquially used as a descriptor for the whole of Tolkien's mythological-fantasy writing over his lifetime.
Many have read "Lead by Niggle" as an autobiographical story about Tolkien's own life, with Niggle's painting of the Great Tree symbolizing Tolkien's creation of the Middle-earth universe. One reason for this interpretation is the sheer scope of both projects as well as the intense detail given to individual parts: Middle-earth, like the Great Tree, is composed of a number of kingdoms and locations, each with their own rich history. Much of the allure of The Hobbit and The Lord of the Rings, for example, comes from the fact that both narratives center on a long journey that allows characters to traverse across nearly the entirety of Middle-earth, interacting with other powers and even species along the way.
When Niggle crosses from the Workhouse to Niggle's Country, he spends the rest of his time before the Mountains working to cultivate the landscape around the Great Tree and surrounding forest. Niggle's work therefore mirrors Tolkien's own, as both are a form of world-making. The story also describes Niggle's original painting as, in many ways, taking on a life of its own, growing and growing until Niggle struggled to finish it. Tolkien's Middle-earth is a similar enterprise, both geographically vast and historically deep. Tolkien himself was also, famously, a terrible procrastinator. While one can certainly read "Leaf by Niggle" as simply an allegorical story about life after death, Tolkien's own biography helps enrich the narrative with a glimpse into his own creative process.
