Three-dimensional filling of a root canal is essential because it completely seals the root canal system, including all lateral canals and irregularities. This prevents entry and growth of bacteria inside the canal, thereby eliminating the risk of reinfection. Proper obturation also stabilizes the canal environment and promotes healing of periapical tissues. Inadequate filling leaves voids that can harbor bacteria, leading to treatment failure. Therefore, the primary goal is to create a hermetic seal to ensure long-term success of root canal therapy.

Reference: Pathways of the Pulp, Cohen & Hargreaves, 12th Edition.