Then Jesus came from Galilee to the Jordan to John, to be baptised by him. John would have prevented Him, saying, “I need to be baptised by You, and do You come to me?” But Jesus answered him, “Let it be so now, for thus it is fitting for us to fulfil all righteousness.” Then he consented. And when Jesus was baptised, immediately He went up from the water, and behold, the heavens were opened to Him, and He saw the Spirit of God descending like a dove and coming to rest on Him; and behold, a voice from heaven said, “This is My beloved Son, with whom I am well pleased.”
The protagonist enters the scene. John the Forerunner previously said he is not worthy to carry (or untie, Luke 3:16) the sandals of this mightier one he is foretelling, then Jesus enters the scene and tells John to baptise Him. John’s response is unsurprising, then. “Why should I baptise You when I’m not even worthy enough to carry Your sandals?” John asks. “Nevertheless,” Jesus says, “I will allow it, in order to fulfil all righteousness.”
Thus, why was Jesus baptised? “To fulfil all righteousness.” That is, to fulfil John’s baptism and have us participate in His Baptism. To vindicate Jesus’ mission, the heavens open up, the Holy Spirit descends, and God the Father confirms His Son. The heavens opened up not only for Jesus, but also for us.
To say our Baptism does nothing (to say it’s a symbol) is to say Christ’s Baptism did nothing—that the opening of the heavens and descent of the Holy Spirit was merely metaphorical speaking and didn’t actually happen. Our Baptism is a participation in the Baptism of Christ. How would we receive the Holy Spirit if the heavens likewise do not open up and the Holy Spirit descends upon us (cf. LC, Part IV, 21)?
In our Baptisms, too, the Father says, “This is My beloved son, with whom I am well pleased.” He is pleased with His baptised sons and daughters on account of Christ.