In summarizing reason why a public judgment must happen on the last day even though individuals are personally judged at death, Gerhard notes that people’s works, good and evil, continue to work even after death. God will reward and punish even this durative good or evil. He notes, for example, that the prophets and apostles still servce the church through their writings.
You see, the good and evil works of the dead are not yet finished. The prophets and apostles are still serving the church with their writings and example. The purveyors of heresies are still corrupting people’s minds with their writings. But when the good and evil works of the dead will be completed on the Day of Judgment along with the world itself, then the ultimate and immutable sentence will be passed.
On Death, § 307.