The primary gutta-percha cone must fit tightly in the apical third because this area is critical for achieving a proper seal and preventing microleakage. A snug fit in the apical third ensures the elimination of voids and reduces the risk of bacterial penetration, which is essential for successful root canal therapy. Additionally, the apical constriction acts as a natural barrier, making it the ideal target for the primary cone to achieve an effective apical seal. Improper fit in this region can compromise the treatment outcome and cause persistent infection.

Reference: Endodontics: Principles and Practice, Mahmoud Torabinejad, 6th Edition.