He doesn't need it in order to breathe and stay alive per se (in the film) but to administer inhaled anesthetics in order to ease the pain left from old injuries suffered at The Pit (mainly to his face I'm guessing?).
I'ma bet it's a combination of all three aspects (mainly the last one) as to why the mask is so important to him in the movie.
Bane wasn't lying when he delivers his "it would be extremely painful" line, which serves as a warning to the other guy as well (double meaning).
What I'm truly curious, though, is as to what exactly does he receive that eases his pain without compromising his awareness and consciousness xD (anesthesia wasn't exactly a favorite subject at med. school so I can't really tell which drug(s) is Bane using).
----- OKAY -----
Now that the whole mask thing is out of the way, I can now focus on your question as to what exactly might have happened to the film version of Bane and since there's no official backstory as to the exact kind of trauma he suffered I'ma speculate based on what I know.
First, I'ma tell you what kind of injuries/illnesses he DOES NOT (or is highly unlikely to) HAVE:
"(...) Take a look: his speed, his ferocity, his training. I see the power of belief. I see the League of Shadows resurging" - Alfred
Bane's physicality is mostly consistent throughout the film. His combat prowess shows he's likely to be in peak condition and is initially superior to Batman (yes he had his issues too to I know)
We see him clinging with one arm at the plane scene, climbing down the sewer chain with ease, beat up people with a very few yet powerful hits, perform special moves during his fight against Batman and even carry him high enough before fracturing his lower back's vertebrae.
Curiously enough, it calls the attention that he always does everything rather slowly, one could argue it's because of his "breathing problem" I will disagree:
He has a cool-toned fair skin with no visible color changes such as:
Pallor, which would suggest anemia (low hemoglobin levels with/without low red blood cells (RBC)).
Cyanosis, which would suggest skin hypoxia and hypoxemia (low di-oxygen levels in his tissues and bloodstream respectively).
Blush, which would suggest polycythemia (high amount of RBC).
Jaundice, which would suggest hyperbilirubinemia (which happens either when RBC die en masse or the person suffers from liver injury/failure).
Earthly, Blackened skin and the like, which would suggest an array of different illnesses such as chronic kidney disease, Addison's disease, insulin resistance (like in diabetes) etc...
The first three I mentioned are often expressed by two important symptoms which are dyspnea (a sense of breathing difficulty during exercise which becomes evident whenever we see a person gasping for air) and chest pain which it can go from palpitations (perceivable accelerated heartbeats) to angor pectoris (as seen in patients with heart attacks and heart failure) depending on the cause.
Bane shows none of this, nor any other correlated symptoms and signs of fatigue, respiratory failure, nor congestive heart failure (no wet nor dry coughing, no wheezing, neck/torso muscles stretching, etc).
As for the latter two skin signs. Should he suffer from a liver, kidney, or pancreatic disorder as mentioned before, it's unlikely that he would be capable of maintaining his muscles in shape or possess such raw energy when delivering his attacks.
A failing liver cannot store glucose for later use, a diabetic patient cannot use said glucose properly and begins creating more by destroying the muscle and fatty tissues which causes the person to become emaciated. Furthermore, a failing liver and kidney can no longer process toxic substances from the bloodstream causing the person to suffer from drowsiness and convulsions (none of which are present in Bane).
BTW the whole throat injury is rubbish. There are no visible injuries in any of the pics above, and if he had inner injuries such as the ones seen in people with laryngotracheal burns through inhalation...
...then his mask would also need to be connected to a respiratory tube inserted into his neck (A.K.A a tracheostomy surgery)
Not to mention it would also make it impossible for him to be such a fluent and well-spoken orator ;)
----- ALRIGHT -----
Now regarding Bane's possible injuries:
LET'S FOCUS ON THE FACE, FIRST
As we can see....
It's perfectly normal!
:)
Okay, terrible joke.
If you're the sensible type you might want to close your eyes and scroll down a bit, I'ma try to censor some of the pics.
"- Many years ago, it was a time of plague, some of the other prisoners attacked Bane. The doctor's fumbling attempts to repair the damage left him in perpetual agony. The mask holds the pain at bay" - Pit prisoner
Two things come to mind:
- LEPRAE, AKA Skin tuberculosis. ("THE PLAGUE")
Considering the Pit - a prison and therefore, very overcrowded place - seems to be located in a 3rd world country with little access to clean water and proper sewage there's a vast array of contagious diseases to appear such as the one I mentioned which causes ulcers that slowly eat the skin (and flesh underneath) away.
Are there other bacteria and viruses capable of doing such grievous damage?
OF COUUURSEEHH
However...
That doesn't seem to affect him right now
Should the person suffer severe and open injuries, the mycobacteria will invade the soft tissues with ease and create something similar as to the cartoon shown above.
which leads me to...
MAXILLOFACIAL TRAUMA or FRACTURES (THE ATTACK)
A well-placed, strong human punch has the ability to fracture the bones in the face, and since Bane pretty much gets mauled by a large prisoner gang he most definitely got punched, kicked, and stomped countless times.
I've seen real-life facial fractures, but you don't normally see large portions of the skin/flesh torn open, but rather a massive swollen face with a dark violet/blueish hue from all the blood located beneath:
I've also seen examples when the flesh does get torn asunder, but this usually happens with high impact trauma, be it passive (deadfall, cliff-fall) or active (wall-fall, vehicular accidents, etc). Human power alone is less likely to make it happen, however it was a gang so there's the possibility still.
Now, whenever trauma takes place, even if we don't quite see a wound in place, it facilitates the access of microorganisms (mostly bacteria) that cause infection.
Add the fracture and you give them an easy access to the bone marrow where they can feast with freshly young RBC and fat. Not to mention an easy access to the bloodstream and cause sepsis later on (which means Bane must be a really resilient s.o.b)
Considering the doctor had to use everything he had at his disposal to save his life and fix his face after the trauma and the place he's at, chances are that Bane suffered an added infection to a severe facial fracture which might have eaten away a part of the skin by the time Ras' shows up...
But then you have Hollywood's lazy representation of his injuries (must be the PG rating thing again)
Still, I will bet my own license that his face's injuries are somewhere in between. Assuming Thalia meeting her father (one wonders how she knew where to find him) and him assaulting the pit took him a few months, you have more than enough time for soft tissue infections to ensue.
It's also important to mention that afflictions such as this have the potential of leaving the local area undergoing chronical inflammation, thus decreasing the resistance of pain due to an often release neurotransmitters, such as one called P-Substance.
Would losing his nose cause Bane severe breathing difficulties? Maybe a bit at first, but as described earlier, his respiratory and circular systems still remain operational.
NOW, LET'S FOCUS ON HIS BODY
The one thing that draws my attention is the large keloid scar on his back, all the way along his spine
The scar goes all the way up to the cervical vertebrae.
Now, I'm not a neurosurgeon, but I'm still going to give it a try...
THE DOCTOR PERFORMED AN EMERGENCY COMPLETE LAMINECTOMY ON BANE IN ORDER TO SAVE HIS LIFE ... THIS can be a big issue:
Bane got punched, kicked, stomped, mangled, etc, everywhere and his back could've also suffered the worst: vertebrae fractures, vertebrae luxation, vertebral disc dislocation, etc.... this can cause one very dangerous thing known as Neurogenic Shock due to spinal cord concussion, crushing, and/or section (the latter two seems far-fetched since Bane can still move).
**Long story short, Neurogenic Shock kills you and the surgery mentioned earlier decompresses the afflicted spinal cord, saving your life. Since there are no x-ray, CT scan, nor MRIs available in the pit, the good doctor had to open the entire laminar tissues surrounding the spine, all the way, to locate the exact cause of Bane's Neurogenic Shock and save his life...
Once more, add the local surgery infections, the chronically inflamed tissue, and you got a recipe for a life of perpetual pain, and... long dissability (Bane's I never escaped line).**
However...
Through the power of will (Or Belief as Alfred put it) he too rose from his injuries (with the help of the mask) overcoming any or most disabilities.
The mask, since it gives him anesthetics periodically not only nullifies pain but any kind of sensation which if summed with any residual damage at his peripheral nerves (either from the surgery itself or the beating he took) means he cannot feel a thing unless the mask comes off and... it could be a reason why he has to move slowly if he no longer feels the floor he's stepping in
Geez... long post, but that's I think Bane's wounds were.
