Why would a surgeon choose to perform brain surgery while the patient is awake?

This may be chosen because the patient's conscious response to stimuli or voluntary actions must be measured to assure the appropriate outcome of the surgery and to influence the actions of the surgeon performing the surgery (as in, if the patient is asked to speak and the surgeon is operating in a part of the brain that controls these functions, if the patient's speech improves, then the surgery is successful - if the patient's speech becomes slurred or inarticulate, then the surgeon may need to do more or may need to stop now).