What is the wisdom behind the prolonged standing on the Day of Resurrection and crossing the Sirat, even though a person knows their destiny after the questioning by the two angels in the grave?

The deceased is shown their seat in Paradise or Hell in the grave. This does not mean that there is no benefit to the gathering (on the Day of Judgment) and the reckoning, because what they see in the grave is their ultimate destiny and their salvation from eternal Hellfire. This does not preclude them seeing their seat in Paradise while still possibly being punished for sins they committed; for seeing the seat relates to Islam or disbelief, and punishment in the grave is not necessarily the punishment resulting in the fire of Hell. Furthermore, the horrors and hardships of the Day of Resurrection may atone for sins.