What is the benefit of the reckoning and intercession on the Day of Judgment, since a person has already known their fate while in their grave?

The deceased's vision of their seat in Paradise or Hell in the grave is part of the torment or bliss of the grave, which is established by texts from the Qur'an and Sunnah (definition). This vision does not contradict accountability and intercession on the Day of Judgment; for intercession has types, including intercession for those for whom Hellfire was decreed, so they do not enter it, and the elevation of ranks for the people of Paradise. As for accountability, it is to manifest God's justice. The disbeliever will be given their book in their left hand and will debate the reckoning, finding in it all they have put forth, as in the verse: "And the book will be placed, and you will see the criminals fearful of that within it, and they will say, 'Oh, woe to us! What is this book that leaves neither a small thing nor a large thing except that it has enumerated it?' And they will find what they did present. And your Lord does not wrong anyone." (18:49). The believer will be given their book in their right hand and will be judged with an easy reckoning, which is the عرض (presentation of deeds), as stated in the Hadith (definition): "Whoever is thoroughly examined in accountability will be tormented."