@softener - Believe it or not, there's no pain in the brain. The patient is usually lightly sedated beforehand and then their scalp is numbed, allowing the surgeon to make the incision.
There are a few reasons why keeping the patient awake might be a good option, the main one being that the patient is able to respond while the operation is being performed. Depending on the area of the brain they're operating, they might ask the patient to try to move occasionally to make sure they're not getting to any areas that might affect motor function. The same can apply for speech. So in many ways it can make major surgery safer.
Also, patients who have not been under general anesthesia tend to heal faster than those who have.