Why be Baptised?

...Because Jesus himself set the example 

Matthew 3:13-17 

(13) Jesus then appeared, arriving at the Jordan River from Galilee. He wanted John to baptize him. 

(14)  John objected, "I'm the one who needs to be baptized, not you!" 

(15)  But Jesus insisted. "Do it. God's work, putting things right all these centuries, is coming together right now in this baptism." So John did it. 

(16)  The moment Jesus came up out of the baptismal waters, the skies opened up and he saw God's Spirit--it looked like a dove--descending and landing on him. 

(17)  And along with the Spirit, a voice: "This is my Son, chosen and marked by my love, delight of my life."