Wisdom is needed to apply knowledge in right order to avoid harm and pursue the chief good, that is holiness. Understanding penetrates the order we use to know, and grasps what is hidden:
“Wherefore I wished (Christian Hope), and understanding was given me: and I called upon God, and the spirit of wisdom came upon me: And I preferred her before kingdoms and thrones, and esteemed riches nothing in comparison of her. Neither did I compare unto her any precious stone: for all gold in comparison of her, is as a little sand, and silver in respect to her shall be counted as clay.I loved her above health and beauty, and chose to have her instead of light: for her light cannot be put out.Now all good things came to me together with her, and innumerable riches through her hands,” (Wis 7:7-11, vulg.)
Wisdom is not given to every plea. “You ask, and receive not; because you ask amiss: that you may consume it on your concupiscences.” (Jas 4:3) We have to be persuaded into God’s order for our lives, that is working in his name, or finding our good in His will. Shaped by the journey we ought to grow in humility and simplicity. And at God’s judgment, those events that occur while we still live, we are given to understand and be wise when we separate from toil and enter contemplation. "Gregory says (Hom. xiv in Ezech.) that it belongs to the contemplative life, “to rest from external action.’” (STh II-II, q. 180, a. 1, Obj. 3) The gift of understanding is practical, and not speculative to our lives: “a good understanding to all that do it.” ( Ps 110 [111]:10, vulg.; cf. Summa Theologica, II-II, q. 8, a. 3, sc.)