The flashback scenes in The Dark Knight Rises explained how Bane got his scars and injuries, but they didn't shed any light as to how he got his mask or why he needed it in the first place. Costume designer Linda Hemming revealed a few tidbits from the movie that ended up getting cut, but she wasn't aware that they weren't in the actual movie because she didn't go see it yet!
Hemming explained that Bane was already wearing an early version of his gas mask and waist belt when he was learning how to fight, Batman-style. And even without a bulletproof vest, Bane still had to wear a waist belt and braces because of his injuries.
If you look at the film, unless they've cut it—and I'm sure they haven't—there's a whole early section for Tom Hardy where he's fighting and being taunted by people. He's got chains on him, and he's standing on a wooden thing while people are attacking him. And in that scene, he's wearing a much more ragged, primitive version of the mask.
I won't elaborate on it too much, because it isn't in the film, but there was another section that showed you why he had the mask and where it came from.
-- Linda Hemming, costume designer
Maybe Christopher Nolan decided to cut those scenes because he wanted to add an air of mystery around Bane's character. Either way, let's just hope those scenes are included in The Dark Knight Rises DVD.
Speaking of deleted scenes, this isn't the first time that supposedly crucial scenes were omitted in the final release. Below you'll find four other key scenes that ended up on the cutting room floor:
Thor. This scene shows Thor and Loki having a brotherly moment and provides a bit more insight into Loki's character.