Praying for the dead is one of the 7 spiritual works of mercy.

We lose nothing, dear brethren, by praying for them; be assured we are rather the gainers, for not only do they pray for us, but more— our charity towards them deepens in our souls our love for God, and makes us thirst the more after virtue and holiness, and wins for us a higher place in heaven and a brighter crown of everlasting glory.…