Why did John the Baptist baptize?
That was his life work-- to preach of the Messiah and to baptize.
John's baptism was "prophecy being fulfilled" before the eyes of that generation.
"...Behold, I send My messenger before thy face, which shall prepare the way before thee. The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.
"John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins.
"And there went out unto him all the land of Judaea, and they of Jerusalem, and all were baptized of him in the river Jordan, confessing their sins." (Mark 1:2-5)
Most people, today, fail to understand what an exciting time it was throughout the land of Judaea, then. John had a large following of thousands of people... waiting for him to point out to them the prophesied "MESSIAH" [the Christ].
By the thousands, people rushed to him to have their sins "buried" in the Jordan river... to have them "washed away"... and to be ready when the Christ finally appeared, to cast out the Roman armies, save them, and restore the kingdom of Israel to their land.
John's baptism of repentance was a "prophetic sign" to the people of that time that the Christ was coming soon.
And it was John's baptism of Jesus that revealed to him that He was the Messiah:
"...John bare record, saying, I saw the Spirit descending from heaven like a dove, and it abode upon Him. And I knew Him not: but He that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on Him, the same is He which baptizeth with the Holy Spirit. And I saw, and bare record that this is the Son of God." (John 1:32-36)
It was John's commission to baptize that eventually revealed to him the One whose way he was preparing. And when John finally revealed it to two of his disciples the next day... the days of his baptizing diminished... and the ministry of the Messiah began.
"...the next day after John stood, and two of his disciples; and looking upon Jesus as He walked, he saith, BEHOLD, THE LAMB OF GOD!
"And the two disciples heard him speak, and they followed Jesus. (verses 35-37)
And from there, the news spread: "...One of the two which heard John speak, and followed him, was Andrew, Simon Peter's brother. He first findeth his own brother Simon, and saith unto him, WE HAVE FOUND THE MESSIAS, which is, being interpreted, the Christ." (verses 40-41)
This "baptism of repentance for the remission of sins" whereby John was commissioned to prepare the way of the Lord, is still the first step to salvation, whereby a person might then receive the "Lord's baptism" of the Holy Spirit:
"...Peter said unto them, REPENT, and BE BAPTIZED EVERY ONE OF YOU in the name of Jesus Christ FOR THE REMISSION OF SINS, and YE SHALL RECEIVE THE GIFT OF THE HOLY SPIRIT." (Acts 2:38)
John's baptism started the ball rolling. It was an exciting time that didn't turn out like the people thought it should. Their "Messiah" was killed... and hopes were dashed.
But, with the resurrection from the dead of the Messiah who now sits at the right hand of the Father in heaven, the call for and the preaching of "repentance and the remission of sins" still goes out today, as fresh and vital as ever, generations later, to those whom God is calling.
"...Thus it is written, and thus it behoved Christ to suffer, and to rise from the dead the third day: and that repentance and remission of sins should be preached in His name AMONG ALL NATIONS, beginning at Jerusalem. And ye are witnesses of these things." (Luke 24:46-48)
John's baptism of repentance for the remission of sins, that once prepared the way of the Lord, is now done in the name of the Lord [the ONE WHO HAS THE POWER TO TRULY BLOT OUT SINS].
That's why John baptized. He prepared the way for the Lord:
"The Word which God sent unto the children of Israel, preaching peace by Jesus Christ: (He is Lord of all:) that Word, I say, ye know, which was published throughout all Judaea, and began from Galilee, after the baptism which John preached; how God anointed Jesus of Nazareth with the Holy Spirit and with Power: who went about doing good, and healing all that were oppressed of the Devil; for God was with Him.
"Now after that John was put in prison, Jesus came into Galilee, preaching the gospel [good news] of the Kingdom of God, and saying, The time is fulfilled, and the Kingdom of God is at hand: REPENT ye, and believe the gospel." (Mark 1:14-15)
John's voice still cries out in the computerized wilderness of our generation, to repent and be baptized for the remission of sins. For the Messiah is coming again.
"And this gospel of the Kingdom shall be preached in ALL THE WORLD FOR A WITNESS UNTO ALL NATIONS; and THEN SHALL THE END COME." (Matt.24:14)
"John answered, saying unto them all, I indeed baptize you with water; but One mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: He shall baptize you with the Holy Spirit [unto salvation] and with fire [unto Judgment]: whose fan is in His hand, and He will purge His floor, and will gather the wheat into His garner; but the chaff He will burn with fire unquenchable." (Luke 3:16-17)
John didn't just "baptize" -- he "preached," too.
Answer also: About six months before Jesus was baptized, John the Baptizer went preaching in the wilderness of Judea saying: "Repent for the kingdm of the heavens has drawn near." (Matthew 3:1,2) People heard what John said and took it to heart. They openly confessed their sins, repented of them, and then came to John to be baptized by him in the Jordan River. See Luke 1:13-16, Acts 13:23,24. More information can be found on Jehovah'/ Witnesses official website.