The Lord Jesus called St. John the Baptist, "the greatest man born of woman."  The Lord preferred this man to all others as his personal herald.  St. Thomas Aquinas (d. 1274), in his commentary on the Gospel of St. Matthew, considers why The Lord chose for his presence to be announced to the world by John:

"Why did Christ wish for his testimony when he had the testimony of his own works?  There are three reasons for this.  First, we are led into the knowledge of spiritual matters by those things that are similar to us.  John 1, 7: 'He came to give testimony to the light.'  And why?  'That all might believe through him.'  Second, because of the wickedness of the Jews, for not only did Christ give testimony for himself, as the Jews said, 'You give testimony for yourself' (John 3, 26), but also another witnessed to him: 'You sent to John and he testified to the truth' (John 5, 33).  Third, to show the equality of Christ to the Father, for as the Father had messengers -- the prophets -- so did Christ.  Luke 1, 76: 'You, child, shall be called the prophet of the Most High.  You shall go before the face of The Lord to prepare his ways.'

"Baptizing was John's special office.  He was the first to baptize, and his baptism was a preparation for the baptism of Christ.  If Christ had added a new rite immediately [in the work of his public life], men would have been scandalized.  Therefore, John went before him to prepare men for his baptism.  John 1, 31: 'That he be made manifest to Israel.'  

"Notice that John did this in his thirtieth year, in which age David was made king, and Joseph undertook the governance of the kingdom of Egypt.  We understand through this that no one should take up an office before the perfect age."