The purpose of narration is to add depth to your world, not tell your audience what's happening.
So many films get this wrong. Rather than open a story visually (the way Up and Sucker Punch do), they have someone explain what's going on... even when it's unnecessary (as they do in the opening of Snow White). Thankfully, Rian Johnson knows how to write. He has the perfect blend of Voice Over (VO) and visual story telling at the beginning of Looper. We can see exactly what's going on, but we don't know why. To help explain this, our narrator sheds light on the world we have been ushered into.
Let me give you an example:
Snow White: We've just seen the queen ill and now she's dead. Cut to a shot of the king grieving and we are told, "The king was inconsolable."
Looper: We see a person suddenly appear on a tarp and get blown away by a guy with a gun. "Time travel will be invented and immediately outlawed..."
In the first example, we already know the king is sad. Telling us this in VO doesn't help us at all. Indeed, the majority of the narration for Snow White is completely pointless. In the second, we now know how these people warp into place and why. We come to understand the intricacies of the world we are shown.
In film, show, don't tell. If it's something that can't be communicated by showing--like someone's reasoning or a ton of history--put the outcome of that idea on the screen (executioners in the past who knock off people from the future) and tell us about why. Do not give us a voice over about something you should have put on the screen.
Your Media Production Mentor