The justification for today's feast is explained by St. Augustine in the Divine Office:
Apart from the most holy solemnity commemorating our Savior's birth, the Church keeps the birthday of no other person except that of John the Baptist. (The feasts of the Immaculate Conception and of the Nativity of the Blessed Virgin had not yet been introduced.) In the case of other saints or of God's chosen ones, the Church, as you know, solemnizes the day on which they were reborn to everlasting beatitude after ending the trials of this life and gloriously triumphing over the world.
For all these the final day of their lives, the day on which they completed their earthly service is honored. But for John the day of his birth, the day on which he began this mortal life is likewise sacred. The reason for this is, of course, that the Lord willed to announce to men His own coming through the Baptist, lest if He appeared suddenly, they would fail to recognize Him. John represented the Old Covenant and the Law. Therefore he preceded the Redeemer, even as the Law preceded and heralded the new dispensation of grace."A Voice Crying Out in the Wilderness"
All four Gospels tell the story of John the Baptist's ministry. Called the Precursor or Forerunner of the Lord, John was born in the town of Judaea, about six months prior to the birth of Our Lord (Luke 1:5). Unlike that of Christ, John's miraculous birth was widely known and greatly celebrated for Elizabeth was thought barren and both she and Zechariah were advanced in years (Luke 1:7).
It is John the Baptist who urged repentance and proclaimed to the world the imminence of the Messiah. Sacred Scripture states that on the day of Christ's baptism, John immediately recognized Jesus as the long awaited Messiah. Despite professing his unworthiness, John baptized Jesus, in acquiescence to Our Savoir's wish. Later, upon hearing of John's imprisonment, Jesus said, "Then why did you go out? To see a prophet? Yes, I tell you, and more than a prophet. This is the one about whom it is written: ‘Behold, I am sending my messenger ahead of you; he will prepare your way before you.’ Amen, I say to you, among those born of women there has been none greater than John the Baptist; yet the least in the kingdom of heaven is greater than he" (Matthew 11:9-11).