Today we pray, not for those who have already made it to heaven (they pray for us) but, for those who are process of purification. I will never understand those who refuse to see the common sense behind the concept of purgatory. There are at least to perspective from which to me it makes absolute sense.
First, which of us if we died right now could say that our hearts are perfectly pure and ready for heaven? No angers, no unhealed wounds, no judgements about others, no sin. Which of us really thinks we wouldn't need a good cleansing?
Secondly, and perhaps more powerfully for me, purgatory explains how God is both loving and just. The person who has spent their life trying to live according to the gospel and the most horrendous criminal who has the last minute dead bed conversion will both get into heaven by the grace and love of God. But, our faith would say, because God is a just God, there is temporal punishment due to sin, and the purification will be different for the two. As any good parent knows you may forgive your child from bad behavior but that doesn't mean they go unpunished.
Do we know the details of precisely how it works, or what it is like? No, that we trust to God. But today's Commemoration of all the Faithful Departed, reminds us that we have a responsibility to pray for all those who have gone before us. I have often observed that we tend to pray most for those who need it the least, those who were the most loving and kind. Perhaps today we can call to mind those who were the most difficult to live with, and pray for them too.