In the Summa Theologica, Aquinas systematically unfolds his Scholastic formulae concerning the relationship of the saints in heaven to the wicked in hell:

Nothing should be denied the blessed that belongs to the perfection of their beatitude. … Wherefore in order that the happiness of the saints may be more delightful to them and that they may render more copious thanks to God for it, they are allowed to see perfectly the sufferings of the damned.

And again,

That the saints may enjoy their beatitude more thoroughly, and give more abundant thanks for it to God, a perfect sight of the punishment of the damned is granted them.

(Quoted by Nietzsche here.)