I'm going to try to make some sense of this, though I don't pretend that this is all why it happened. Beyond having attention drawn to him by supernatural events, an obvious, unmistakable fact that is not dependent on cultural context(I mean, a time traveller from a completely unevangelized culture would find that interesting), we also have John baptizing him. Now, if I were to read this without any sort of context or knowledge of Hebrew history, I would say that John's baptism signifies a sort of “group effort.” I would say that God has lots of folks walking around, agents if you will, and there happen to be two of them here doing...holy things. Like God's people do. You know, baptizing, wearing robes, saying “thee” and “thou” a lot, that kind of stuff. Jesus and John were some real good guys. And that's what history is- some bad people, some good ones, and you choose whether you wanna be good or bad, and hopefully you'll choose right. Or something wretched like that.
Two things. First, John says what's obvious to us, but not perhaps to the people watching, or someone who's never heard of Jesus: “You're better than me. I shouldn't be baptizing you, you should be baptizing me. I'm not worthy of you.”
Second, the people watching are Jews, temple-going, Torah-reading, tradition-saturated, children of Abraham. Of their history, practice, heritage, and corporate dealings with God, they are richly aware. John is one of these people God sends, one of these prophets that is given the mantle of speaking for God. To people. Of acting and speaking in such a way that displays God's opinions, his plans, his character, his judgments, etc. These are God's mouthpieces to Israel. John, a full-on, prophet in the style of the good old days and the mold of Elijah is here to preach repentance, baptize, and perhaps do some of what prophets do best: call attention to sin.
So we have a man who is dubbed(later) the “greatest man born of women” by Jesus. In his time the greatest prophet, the last Old Covenant “mouthpiece” the one who has, evidently, the “spirit of Elijah,” which I take to mean something like, “he's cut from the same cloth” or “he has a calling that is as great or greater than Elijah's.” It's a superlative, which would be something like saying “so-and-so is the greatest president since Abraham Lincoln.” And here he is baptizing Jesus. Calling himself unworthy to tie Jesus' shoes.
What I think God intends to happen in the minds of Jewish onlookers, and by extension in us who are reading about it, is something like this: “Our Fathers, our Law, our sacrifices, our prophets, our entire testimony of God's work among us has come down to this: Jesus. Here is John baptizing him- it's as if the entire canon of saints from Abraham, Moses, David, Isaiah, all are being drawn towards the baptized one as iron filings are to a magnet. The entire divine revelation up to this point is commending Jesus as superior than itself, more worthy, a greater King, Prophet, Judge, Priest, Law, and Temple, through the Baptist. Indeed the Baptist himself is greater than everyone up to this point for this reason-- he's the one who gets to step aside and “diminish” in Jesus' presence, having prepared the way for him and made his path straight. The baptism of John is God's way of witnessing to Jews that the mantle of all that has happened up to now is being placed on Jesus' shoulders. Through John, the entire Old Testament is baptizing Jesus.
This baptism is to “fulfill all righteousness(Matt 3:15).” That is: 1) inaugurate Jesus' earthly ministry which will lead to the Cross and through it the Resurrection(which is his righteousness), and 2) give this whole event a wide “viewership,” a kick-off that will get people's attention, so that hopefully, they'll still be paying attention when he gets crucified and they'll first “believe” and then “abide”(which is the the Church's righteousness). God wants the watching Jews to get this, that Jesus is the one for whom the Old Covenant has been “preparing the way.” He wants these people to continue to read their Bibles, the Law and the Prophets. And he wants them to read it correctly. He wants the history of the Kingdom of God- the Hebrew nation- to be understood as having one purpose- to make a straight path for Jesus. So that when Jesus does what he's about to do, namely gets crucified, they won't be stumbling around in the dark trying to figure out how to reconcile this supposed “son of God” with all that they already know. They'll see the Cross to which his ministry led as the centerpiece of their own history, and they'll have new prophets explaining to them(with words and actions, as prophets always do) “all that the (old) prophets have spoken.”
Of course a whole bunch of them don't listen anyway. But if we listen, we can probably be rooted in a historic salvation rather than just a theoretical one. After all, that's why this whole thing took place.
To all who did receive him, who believed in his name, he gave the right to become children of God.