So without further ado, here's Aquinas on why Christ rose from the dead. . .
It behooved Christ to rise again, for five reasons. First of all; for the commendation of Divine Justice, to which it belongs to exalt them who humble themselves for God's sake, according to Lk. 1:52: "He hath put down the mighty from their seat, and hath exalted the humble." Consequently, because Christ humbled Himself even to the death of the Cross, from love and obedience to God, it behooved Him to be uplifted by God to a glorious resurrection; hence it is said in His Person (Psalm 138:2): "Thou hast known," i.e. approved, "my sitting down," i.e. My humiliation and Passion, "and my rising up," i.e. My glorification in the resurrection; as the gloss expounds.
Secondly, for our instruction in the faith, since our belief in Christ's Godhead is confirmed by His rising again, because, according to 2 Cor. 13:4, "although He was crucified through weakness, yet He liveth by the power of God." And therefore it is written (1 Corinthians 15:14): "If Christ be not risen again, then is our preaching vain, and our [Vulg.: 'your'] faith is also vain": and (Psalm 29:10): "What profit is there in my blood?" that is, in the shedding of My blood, "while I go down," as by various degrees of evils, "into corruption?" As though He were to answer: "None. 'For if I do not at once rise again but My body be corrupted, I shall preach to no one, I shall gain no one,'" as the gloss expounds.
Thirdly, for the raising of our hope, since through seeing Christ, who is our head, rise again, we hope that we likewise shall rise again. Hence it is written (1 Corinthians 15:12): "Now if Christ be preached that He rose from the dead, how do some among you say, that there is no resurrection of the dead?" And (Job 19:25,27): "I know," that is with certainty of faith, "that my Redeemer," i.e. Christ, "liveth," having risen from the dead; "and" therefore "in the last day I shall rise out of the earth . . . this my hope is laid up in my bosom."
Fourthly, to set in order the lives of the faithful: according to Rm. 6:4: "As Christ is risen from the dead by the glory of the Father, so we also may walk in newness of life": and further on; "Christ rising from the dead dieth now no more; so do you also reckon that you are dead to sin, but alive to God."
Fifthly, in order to complete the work of our salvation: because, just as for this reason did He endure evil things in dying that He might deliver us from evil, so was He glorified in rising again in order to advance us towards good things; according to Rm. 4:25: "He was delivered up for our sins, and rose again for our justification."
[From Summa Theologica, III, Q. 53, Art. 1]