After introducing God, John introduces himself. John has a calling from God — “to bear witness of the Light.” (1:6) According to the Merriam-Webster dictionary (my favorite dictionary to use), a witness is an “attestation of a fact” or “one that gives evidence”. So in essence, John’s purpose is to show us that Jesus is legit and a light (hence why he wrote the gospel in the first place). That way, we can believe in Jesus (which I’ll get to that later).
One notable part is that John emphasizes that he himself is NOT the light — Jesus is (1:8-9). John is basically a messenger (like a news reporter) to let people know who Jesus is. That way, they can experience the light (guidance).