Objection 1. It would seem unbecoming that Christ should pray. For, as Damascene says (De Fide Orth. iii, 24), "prayer is the asking for becoming things from God." But since Christ could do all things, it does not seem becoming to Him to ask anything from anyone. Therefore it does not seem fitting that Christ should pray.
Objection 2. Further, we need not ask in prayer for what we know for certain will happen; thus, we do not pray that the sun may rise tomorrow. Nor is it fitting that anyone should ask in prayer for what he knows will not happen. But Christ in all things knew what would happen. Therefore it was not fitting that He should ask anything in prayer.
Objection 3. Further, Damascene says (De Fide Orth. iii, 24) that "prayer is the raising up of the mind to God." Now Christ's mind needed no uplifting to God, since His mind was always united to God, not only by the union of the hypostasis, but by the fruition of beatitude. Therefore it was not fitting that Christ should pray.
On the contrary, It is written (Luke 6:12): "And it came to pass in those days, that He went out into a mountain, and He passed the whole night in the prayer of God."
I answer that, As was said in the II-II, 83, 1,2, prayer is the unfolding of our will to God, that He may fulfill it. If, therefore, there had been but one will in Christ, viz. the Divine, it would nowise belong to Him to pray, since the Divine will of itself is effective of whatever He wishes by it, according to Psalm 134:6: "Whatsoever the Lord pleased, He hath done." But because the Divine and the human wills are distinct in Christ, and the human will of itself is not efficacious enough to do what it wishes, except by Divine power, hence to pray belongs to Christ as man and as having a human will.
Reply to Objection 1. Christ as God and not as man was able to carry out all He wished, since as man He was not omnipotent, as stated above (13, 1). Nevertheless being both God and man, He wished to offer prayers to the Father, not as though He were incompetent, but for our instruction. First, that He might show Himself to be from the Father; hence He says (John 11:42): "Because of the people who stand about I have said it" (i.e. the words of the prayer) "that they may believe that Thou hast sent Me." Hence Hilary says (De Trin. x): "He did not need prayer. It was for us He prayed, lest the Son should be unknown." Secondly, to give us an example of prayer; hence Ambrose says (on Luke 6:12): "Be not deceived, nor think that the Son of God prays as a weakling, in order to beseech what He cannot effect. For the Author of power, the Master of obedience persuades us to the precepts of virtue by His example." Hence Augustine says (Tract. civ in Joan.): "Our Lord in the form of a servant could have prayed in silence, if need be, but He wished to show Himself a suppliant of the Father, in such sort as to bear in mind that He was our Teacher."
Reply to Objection 2. Amongst the other things which He knew would happen, He knew that some would be brought about by His prayer; and for these He not unbecomingly besought God.
Reply to Objection 3. To rise is nothing more than to move towards what is above. Now movement is taken in two ways, as is said De Anima iii, 7; first, strictly, according as it implies the passing from potentiality to act, inasmuch as it is the act of something imperfect, and thus to rise pertains to what is potentially and not actually above. Now in this sense, as Damascene says (De Fide Orth. iii, 24), "the human mind of Christ did not need to rise to God, since it was ever united to God both by personal being and by the blessed vision." Secondly, movement signifies the act of something perfect, i.e. something existing in act, as to understand and to feel are called movements; and in this sense the mind of Christ was always raised up to God, since He was always contemplating Him as existing above Himself.
Source: Summa Theologica (New Advent)