ELI 5, why do some movies open and close with the letterbox, but throughout fill the entire screen?

Just finished watching Heat, and I noticed that the film opened with the black bars on top and bottom, then after a couple minutes the film went full screen. What's the deal with that? I know this also happened with JFK, and other movies I've seen in the past. Any idea why this is a thing? Thanks.

Submitted September 05, 2017 at 05:16PM by TheSheikYerbouti http://ift.tt/2f1qVd2