What was the purpose of the Baptism of Water versus a Baptism of Repentance?
The Baptism of Water was a ceremonial cleansing used in the Old Testament. It was something the people understood that was similar to “baptism” as a whole.
The purpose of the water regarding them being baptized again is to signify making the commitment to Christ when He comes. What we see further on is the connection between when Christ comes to be baptized by John, the question many people ask is “Why did Jesus even have to be baptized?”
Jesus’ baptism ultimately signifies Him taking on the sins of the world in order for Him to be our substitutionary atonement so that we might live through Him.
Dr Arnold Fruchtenbaum calls this an eschatological anticipation of what is to come. This (baptism) is simply preparing them, ready to receive their Messiah.
In Acts 19, there were some people baptized unto the baptism of repentance. But they had not seen the Christ or heard of the Christ. When Paul comes across them, he asks if they have received the Messiah. Their reply, is that they didn’t know there was a Holy Spirit at all. So when they’re baptized at that moment (by Paul) they receive the Holy Spirit and in doing so they now receive the Messiah.