Instead of having the droids play the message (which would just look like a map and not seem that big) we need someone to explain what it is and give Finn a reason to team with Rey. And then, because we don't have a fighter pilot, we need to bring him back so we can have someone we care about bombing the Death Star Planet.

In fact, he was supposed to die in that opening scene.

http://www.gq.com/story/star-wars-the-force-awakens-character-death