Belief in the Last Day is among the pillars of faith without which faith is not complete. Promise (waʿd) and threat (waʿīd) are the ultimate purpose of the Last Day; whatever is besides them is like a prelude.
By promise is meant what Allah Exalted is He has informed of the obedient deserving everlasting reward in the Gardens of Bliss.
By threat is meant what Allah Exalted is He has informed of the disobedient deserving everlasting punishment in the torment of Hellfire.
The Qur’an is filled with mention of that, and that it is recompense for deeds. The Commander of the Faithful (Peace be Upon Him) mentioned the wisdom in that, saying: “Indeed, Allah has placed reward upon obedience to Him and punishment upon disobedience to Him, to keep His servants away from His vengeance and to drive them toward His Paradise.”
Its meaning is that promise and threat serve as a deterrent from committing sins and enormities, from entering into injustice and transgression, and from engaging in any impurity of disobedience; and as a motive toward performing good deeds, purifying the soul, cultivating noble character, beautifying actions, and giving thanks for blessings. That is a cause for the pleasure of Allah Exalted is He, entry into His mercy, and safety from His punishment.