Objection 1. It would seem that it was not fitting that John should baptize. For every sacramental rite belongs to some law. But John did not introduce a new law. Therefore it was not fitting that he should introduce the new rite of baptism.
Objection 2. Further, John "was sent by God... for a witness" (John 1:6-7) as a prophet; according to Luke 1:76: "Thou, child, shalt be called the prophet of the Highest." But the prophets who lived before Christ did not introduce any new rite, but persuaded men to observe the rites of the Law. as is clearly stated Malachi 4:4: "Remember the law of Moses My servant." Therefore neither should John have introduced a new rite of baptism.
Objection 3. Further, when there is too much of anything, nothing should be added to it. But the Jews observed a superfluity of baptisms; for it is written (Mark 7:3-4) that "the Pharisees and all the Jews eat not without often washing their hands... and when they come from the market, unless they be washed, they eat not; and many other things there are that have been delivered to them to observe, the washings of cups and of pots, and of brazen vessels, and of beds." Therefore it was unfitting that John should baptize.
On the contrary is the authority of Scripture (Matthew 3:5-6), which, after stating the holiness of John, adds many went out to him, "and were baptized in the Jordan."
I answer that, It was fitting for John to baptize, for four reasons: first, it was necessary for Christ to be baptized by John, in order that He might sanctify baptism; as Augustine observes, super Joan. (Tract. xiii in Joan.).
Secondly, that Christ might be manifested. Whence John himself says (John 1:31): "That He," i.e. Christ, "may be made manifest in Israel, therefore am I come baptizing with water." For he announced Christ to the crowds that gathered around him; which was thus done much more easily than if he had gone in search of each individual, as Chrysostom observes, commenting on St. John (Hom. x in Matth.).
Thirdly, that by his baptism he might accustom men to the baptism of Christ; wherefore Gregory says in a homily (Hom. vii in Evang.) that therefore did John baptize, "that, being consistent with his office of precursor, as he had preceded our Lord in birth, so he might also by baptizing precede Him who was about to baptize."
Fourthly, that by persuading men to do penance, he might prepare men to receive worthily the baptism of Christ. Wherefore Bede [Cf. Scot. Erig. in Joan. iii, 24] says that "the baptism of John was as profitable before the baptism of Christ, as instruction in the faith profits the catechumens not yet baptized. For just as he preached penance, and foretold the baptism of Christ, and drew men to the knowledge of the Truth that hath appeared to the world, so do the ministers of the Church, after instructing men, chide them for their sins, and lastly promise them forgiveness in the baptism of Christ."
Reply to Objection 1. The baptism of John was not a sacrament properly so called [per se, but a kind of sacramental, preparatory to the baptism of Christ. Consequently, in a way, it belonged to the law of Christ, but not to the law of Moses.
Reply to Objection 2. John was not only a prophet, but "more than a prophet," as stated Matthew 11:9: for he was the term of the Law and the beginning of the Gospel. Therefore it was in his province to lead men, both by word and deed, to the law of Christ rather than to the observance of the Old Law.
Reply to Objection 3. Those baptisms of the Pharisees were vain, being ordered merely unto carnal cleanliness. But the baptism of John was ordered unto spiritual cleanliness, since it led men to do penance, as stated above.
Source: Summa Theologica (New Advent)