There are at least four reasons why we pray for the departed.
1] We pray for the departed because in the most ancient understanding of the church, those who have departed this world are still alive.
"For He is not the God of the dead but of the living, for all live to Him." Lk 20:38.
Therefore, since the departed still live, we continue to interact with them by faith, even though we cannot see them with our physical eyes.
2] We also pray for the departed because Scripture instructs us that those who have passed on are presently watching us in the same way as witnesses watch the unfolding of an event. They can see us and interact with us. As the book of Hebrews says:
Therefore we also, since we are surrounded by so great a cloud of witnesses, let us lay aside every weight, and the sin which so easily ensnares us, and let us run with endurance the race that is set before us… Heb 12:1.
As we see it, creation is a one story universe, totally interconnected for those who can see it by faith.
3] We pray for the departed because we know that those who have passed on not only see us, but that they continue to care for us, and we for them.
"Then he (the rich man who had died) cried and said, 'Father Abraham, have mercy on me, and send Lazarus that he may dip the tip of his finger in water and cool my tongue; for I am tormented in this flame.'
"But Abraham said, 'Son, remember that in your lifetime you received your good things, and likewise Lazarus evil things; but now he is comforted and you are tormented. 'And besides all this, between us and you there is a great gulf fixed, so that those who want to pass from here to you cannot, nor can those from there pass to us.'
"Then he said, 'I beg you therefore, father, that you would send him to my father's house, 'for I have five brothers, that he may testify to them, lest they also come to this place of torment.' Lk 16:24-28
As we can see in the above parable, love does not stop with one’s passing on to the next life. Just as we care and pray for someone before they pass on; we continue care and pray for them now and they for us. In this parable, it is obvious that the entreaties of the rich man availed little, nevertheless, the scriptures promise that the prayers of the righteous are heard:
“The effective, fervent prayer of a righteous man avails much”… Ja 5: 16.
4] Lastly, we pray for those who have passed because we are commanded by the apostle to pray for everyone without exclusion.
Therefore, I exhort first of all that supplications, prayers, intercessions, and giving of thanks be made for all men… 1 Tim 2:1.
For this same reason, we ask them to pray for us.