Objection 1. It would seem that understanding, considered as a gift of the Holy Ghost, is not practical, but only speculative. For, according to Gregory (Moral. i, 32), "understanding penetrates certain more exalted things." But the practical intellect is occupied, not with exalted, but with inferior things, viz. singulars, about which actions are concerned. Therefore understanding, considered as a gift, is not practical.
Objection 2. Further, the gift of understanding is something more excellent than the intellectual virtue of understanding. But the intellectual virtue of understanding is concerned with none but necessary things, according to the Philosopher (Ethic. vi, 6). Much more, therefore, is the gift of understanding concerned with none but necessary matters. Now the practical intellect is not about necessary things, but about things which may be otherwise than they are, and which may result from man's activity. Therefore the gift of understanding is not practical.
Objection 3. Further, the gift of understanding enlightens the mind in matters which surpass natural reason. Now human activities, with which the practical intellect is concerned, do not surpass natural reason, which is the directing principle in matters of action, as was made clear above (I-II, 58, 2; I-II, 71, 6). Therefore the gift of understanding is not practical.
On the contrary, It is written (Psalm 110:10): "A good understanding to all that do it."
I answer that, As stated above (Article 2), the gift of understanding is not only about those things which come under faith first and principally, but also about all things subordinate to faith. Now good actions have a certain relationship to faith: since "faith worketh through charity," according to the Apostle (Galatians 5:6). Hence the gift of understanding extends also to certain actions, not as though these were its principal object, but in so far as the rule of our actions is the eternal law, to which the higher reason, which is perfected by the gift of understanding, adheres by contemplating and consulting it, as Augustine states (De Trin. xii, 7).
Reply to Objection 1. The things with which human actions are concerned are not surpassingly exalted considered in themselves, but, as referred to the rule of the eternal law, and to the end of Divine happiness, they are exalted so that they can be the matter of understanding.
Reply to Objection 2. The excellence of the gift of understanding consists precisely in its considering eternal or necessary matters, not only as they are rules of human actions, because a cognitive virtue is the more excellent, according to the greater extent of its object.
Reply to Objection 3. The rule of human actions is the human reason and the eternal law, as stated above (I-II, 71, 6). Now the eternal law surpasses human reason: so that the knowledge of human actions, as ruled by the eternal law, surpasses the natural reason, and requires the supernatural light of a gift of the Holy Ghost.
Source: Summa Theologica (New Advent)