Why was Yeshua baptized?
Have you ever considered Yeshua's water immersion as atoning? Why did thousands of Jews flock to John to be baptized? (At the time, water immersion was a very Jewish concept.) To repent, of course (Matt. 3:2). This is why when the sinless one came to John, he said, “I need to be baptized by you, and do you come to me?” (Matt. 3:14). We know that John prophetically knew he was sinless, because he called him, “Lamb of God, who takes away the sin of the world!” (John 1:29) Yeshua's response to John was that this act “is proper for us to do this to fulfill all righteousness” (Matt.3:15).
What does that mean?
It could not mean to fulfill the law because no law required baptism. While “fulfill” generally refers to prophecy, there are no clear connections to baptism in prophecy.  I think it was in fulfillment of the atonement prophecies that we see in Isaiah 53. He identifies with the sin of Israel (and the world) through baptism. Thus he bookends his ministry by first identifying with the sin of Israel and then fully taking it on himself on the cross. “This act foreshadows the time on the cross when he will die for the sins of the people of Israel and indeed for the sins of all those who are his.” 
In the first instance, God affirms him saying, “This is my Son, whom I love; with him, I am well pleased” (Mt 3:17), while in the second, the Father and Yeshua are separated to the point that Yeshua agonizes in the garden over going to the cross. He had no fear of pain or humiliation. He already chose that when he came in the form of a servant, sinful man (Phil. 2:5-8). He agonized over the coming separation when we would quote the psalmist while on the cross, saying, “My God, my God, why have you forsaken me?” (Matt. 27:46).
Barton gives six reasons for this baptism, but only the first is needed. “To confess sin on behalf of the nation, as Isaiah, Ezra, and Nehemiah had done.”  Craig Keener agrees, “This baptism hence probably represents Jesus’ ultimate identification with Israel at the climactic stage in its history: confessing its sins to prepare for the kingdom (3:2, 6).” 
Let’s also focus on the joy of the Father at His immersion. “The phrase ‘in whom I am well pleased’ means that the Father takes great delight, pleasure, and satisfaction in the Son. The verb in Greek conveys that God’s pleasure in the Son is constant. He has always taken pleasure in his Son.”
Why was God pleased at that moment? The plan of Israel’s redemption has commenced. We are one step closer to the joy of Shavuot (Pentecost) when 3,000 Jewish men plus women and children would confess faith and solidify this faith by going into the waters of immersion, just like Yeshua, as there were over 100 immersion take surrounding the temple. WHAT JOY THERE MUST HAVE BEEN! Soon, these Jewish disciples would take his message to the “ends of the earth” (Acts 1:8), to the nations.
But there is another reason. The words “This is my Son” are meant to bring us back to Psalm 2, where Yeshua says, “He said to me, “You are my son; today I have become your father” (v. 7). This is a Messianic prophecy about when the Messiah would be coordinated in the midst of the defeated nations in Jerusalem. “Kiss the son lest he be angry” (v. 12). The kings will submit to him. And he will reign from Israel, “I have installed my king on Zion, my holy mountain” (v. 6). It points to Rev. 11:15 when the kingdoms of this world are given to Yeshua.
“The kingdom of the world has become the kingdom of our Lord and of his Messiah, and he will reign for ever and ever.”
Another point that this baptism was truly the beginning of his martyrdom on behalf of Israel is the similarity to Stephen’s martyrdom. Heaven opens, but instead of the Father speaking, it is the risen Son who honors him with a standing ovation.
“But Stephen, full of the Holy Spirit, looked up to heaven and saw the glory of God, and Jesus standing at the right hand of God. ‘Look,’ he said, ‘I see heaven open and the Son of Man standing at the right hand of God’” (Ac 7:55–56).
Lastly, God’s voice is seen as a ‘restarting’ of prophetic ministry. For 400 years, God has been silent. While the rabbis spoke of the Bat Kol (the voice of the daughter) as a prophetic voice during the intertestamental period, it is “More likely the voice is a sign that divine communication with Israel is resuming.” 
Yeshua was immersed at the beginning of his suffering, which culminated in the crucifixion, like book ends. How did he suffer in the water? By identifying with Israel’s sin. He goes into the water and then sheds his blood. Pilate washes his hands with water to be “innocent of this man’s blood” (Matt. 27:24). When Yeshua died, both water and blood flowed from this side (John 19:34). Gerald Borchert, knowing that John is full of symbolism writes “I have heard it argued strongly that the water here could equally represent the water of baptism.”
Yeshua identified with Israel’s sin through baptism and then took Israel’s sin upon himself through crucifixion.
“He identifies with the sinful people of Israel, and he identifies with their sin, because he is coming to be both the final sacrifice and the final high priest (Heb 8–10). ...This act of exchange, in which Jesus takes our sin and gives to us his righteousness, is depicted symbolically beforehand when he is baptized by John.”