Why do we pray for the repose of a soul?

Recently, I have attended several funerals. I have heard this often, but have never cared to understand what it meant. Now I do. When it is announced that someone has died, the priest usually tells us to pray for the repose of _____'s soul. What does that mean exactly?

It means that we are asking God that if ________ is in purgatory, his suffering might be quickly completed and he be brought into heavenly glory as quickly as possible.

Recommended reading:

The Roots of Purgatory
The Biblical Doctrine of Purgatory by Patrick Madrid

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