These are good questions, shared by many others, and they touch upon a long tradition in the Church of not only remembering those who have gone on before us, but also commending them to God’s loving mercy.
There is a mortuary chapel, built in the 1870s, that stands in the center of the cemetery on the grounds of the now-closed Jesuit seminary at Woodstock, Maryland. Inscribed over the chapel door are these words (in Latin): “Here the Society of Jesus cherishes as keepsakes for heaven the ashes of the dear sons she has brought forth."
The cherished remains are “keepsakes for heaven;" that’s why we visit cemeteries. The prayerful remembrances — Masses, special prayers for the dead, Rosaries, Stations of the Cross, and other devotional remembrances — are, for the most part, petitions to God. They ask that the departed believer may be united in an eternal embrace with the triune God — Father, Son, and Holy Spirit — in heaven. For that goal was each of us created, for that purpose did Christ die and rise again, and for that end do we pray when we commend the souls of “the faithful departed" to God.
But, you may be wondering, why are they not united with God immediately upon leaving this world? Well, that question relates to our readiness, our preparedness, our freedom from sin, and our satisfaction of what the Church refers to as “temporal punishment due to sin" in order that the union of a human being — a “mere mortal" — with the sinless God is possible.
What is this temporal punishment? Let me suggest that it is a condition of “unreadiness" for eternal union with the Holy Trinity. In one sense you are ready, because you’ve expressed your sorrow and your sins have been forgiven. But in all probability you’re not quite ready, because your love of God at the moment of death may be less than wholehearted, less than perfect. Spiritually, you are right with God (your sins have been forgiven), but you need a bit of tidying up before being taken fully into God’s loving and eternal embrace.
Purgatory is the cure for your condition of unreadiness. It’s the process of purification.
Hence, we pray for the dead to beg God to move that process along. It all relates to God’s love and grace, that they may enfold the souls of the departed and keep them eternally secure.
The Mass, of course, is the absolute best prayer for the dead. It makes the redemptive sacrifice of Christ present again on the altar and, in God’s gracious providence, allows you to ask that this redemptive power be applied to the one for whom you pray.
You didn’t ask, but let me add to this the truth that those who have died can pray for you. They are with God and can pray to God for you. Similarly, your prayers for anyone who is already fully and eternally united to God in love are not wasted. All prayer is powerful, and in this case, your prayer will return in the form of unknown and unpredictable blessings on you and your world.
See Prayer For the Dead Here