Orthodox Priests pray for forgiveness of sins during the Trisagion Prayers for the newly departed:
p. 184 - Again we pray for the repose of the soul of the servant of God, N., departed this life; and that Thou wilt pardon his/her every transgression, both voluntary and involuntary.
p. 185 - That the Lord God will establish his/her soul where the Just repost; the mercies of God, the kingdom of heaven, and remission of his sins, let us ask of Christ, our Immortal King and our God.
Service Book of the Holy Eastern Orthodox Catholic and Apostolic Church, Antiochian Orthodox Christian Archdiocese of North America, Tenth Edition, 1997, p. 184-185.
So, why would Orthodox Priests pray for the pardon of his/her every transgression, if these newly departed souls could not benefit from this prayer?
Prayer to God has the benefit of not being restricted by time in its efficacy. Since He (beyond exists) beyond the limits of linear time, a prayer to Him can have benefit even for those who have already passed on. It is not a fruitless exercise.
However, from the POV of the deceased, their time to repent is over at death. As a simple spirit with no body, they are predisposed to act in death as they have in life, and do not have the benefit of the changability of the body to justify repentance (i.e. the distinction between human and angelic sin - when the angels rebelled they had no chance to repent, since as simple spirits they were aligned purely without distraction; we, on the other hand, are distracted by our senses, feelings, limited scope, etc.). We become in a manner of speaking like the angels - i.e. we are at that moment only spirit - and, like them, have a straight trajectory to follow.
Our hope rests in the Church militant, the living followers of Christ, that they may pray for us and our benefit, to plead before God (as the angels, saints, and, most of all, the Theotokos do) for mercy on our souls.