I realize this question is somewhat vacuous, since one could ask:
"Why do the OT prophets exist? Why wasn't the story just: Eve + Adam eat from the tree of knowledge of good + evil. Then, God, immediately sends Christ down, who dies for humanity's sins, QED."
Nevertheless, what is the purpose of John the Baptist's existence / ministry, given that it occured in the same time period as Christ?
There is a very significant even of John Baptizing Jesus:
13 Then Jesus came from Galilee to the Jordan to be baptized by John. 14 But John tried to deter him, saying, “I need to be baptized by you, and do you come to me?” 15 Jesus replied, “Let it be so now; it is proper for us to do this to fulfill all righteousness.” Then John consented. 16 As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him. 17 And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.”
Besides that, what is John the Baptist's contribution/significance/utility to Christianity?