Anyways Bane wears the mask because when he was protecting Talia (Miranda Tate) he sacrified his health and safety into making sure she escapes and he was attacked because he was born and raised in a prison. But in the comics the original reason was an experiment that was tested on Bane which left him almost dead but the MASK gives him life.
1. Based on the scar that can be seen on the back of his neck, it seems reasonable to deduce that he has had some sort of extensive cervical fusion as a result of significant damage to his cervical spine.
2. This kind of operation can sometimes cause residual intense pain. In other words, it hurts ALL THE TIME.
3. In the movie, the doctor can be heard speaking about a prisoner who became addicted to morphine as a result of a terrible accident.
4. The scene showing Talia Al Ghul escaping also shows Bane fighting off her would-be attackers. It can be assumed that he was overrun and severely beaten. THIS is the accident the doctor speaks of.
5. The tubing on the front of the mask indicates some sort of inhalation system. Also, when Batman damages the mask during his and Bane’s second fight, pressurization can be heard leaking from Bane mask.
Conclusion: It is reasonable to believe that Bane suffered terrible damage to his spine during his fight to defend Talia as she escaped. The doctors were able to save his life, but only by an operation consisting of extensive cervical fusion. The operation was not 100% successful and now causes Bane constant and excruciating pain. Bane now wears the mask in order to directly inhale a constant flow of anesthetic in order to make the pain bearable. So the Bane mask is very important to Bane.
NOTE: The only way for a mask like this to work in real life is if the anesthetic were freeze-dried and kept that way during use.