Furthermore, the "closure" represented a disconnect with Boo. If you watch how the story progresses, once they are no longer able to exchange these items with Boo, his existence doesn't really come up for chapters. In fact, the tone and atmosphere of the novel shifts completely from the children's very immature pursuit of this innocent man to the adults' very real pursuit of a different innocent man. Without that knothole being closed up by Mr. Nathan Radley, we as a reading audience might have continued to hear from Scout from her very self-centered account on their continued Boo Radley curiosities. As it is, the closure of this chapter to their lives meant their curiosity had to take life somewhere else. This somewhere else happened to be the courtroom in Tom Robinson's case.
I think this closure symbolized a closure to the reality of childhood which includes a realm of imagination and a belief in the good of people.
The knothole in the Radley oak tree serves as the secret communication outlet between Boo and the Finch children. It becomes important to the story because without the knothole, Jem and Scout may never have been able to make any kind of contact with Boo, nor would they have ever realized that he was not the ghoulish character of neighborhood lore. His gifts to them slowly broke down the children's fears of him, and they eventually realized that he wanted to be their friend. The cementing of the knothole by the elder Radley ended their indirect line of contact. It also served as a wake-up call to Jem. After Atticus explained to him that the tree was healthy, Jem realized that he had been lied to by Radley, and he reasoned that the only reason was out of pure meanness--a trait not restricted to children but to mature adults as well.