The most commonsense answer to why we pray for the dead is this…
- It comes naturally to man to continue to think of his passed away loved ones.
- It comes naturally to man to continue to have concern as to where they are and to things beyond the grave.
- It comes naturally to man to take recourse for their well-being by way of prayer and petition knowing that good and evil are very real.
These three things inform us that we right well have a continued responsiblity to exercise charity for our brethren. True, we cannot bring a dead man a drink of water, like the woman brought to Christ at the well, but we can exercise a service at least by way of intention. For the intention to be fufilled it is best to vocalise it. And it we offer our breath in a little s… sacramental nature. The breath is the material part of the action. The form is the prayer, especially so if it is the prayer of the Church, “Fidelium animae, per misericordiam Dei, requiescant in pace.” (I am no Theologian but I got that much down anyway) Regardless, when man is baptised he enters into the priesthood of Christ (not in the fullest way of course as in the case of a bishop). And, interceding for the dead is part of the priestly-office of the baptised man (and woman… do get into tiffy over proper gender in English).
There is the old book of Maccabees answer. That is a very, very good answer. But, we musn’t fail to observe human nature, fallen as it is, with our intellect by way of prayer and philosophy.
As a bonus… one can think of the Mercy of God in terms of Mary, since She is the Mother of All-Graces, and also Mother of Mercy, Sweetness, and Hope.
I am sure someone else has something to add.
EDIT: I just noticed that someone else had put a very good answer using 1, 2, and 3. Cool… That and prayers are not bound by earthly time is correct.