Why is necessary to have during awake brain surgery head fixed in a frame? when patient woke up, feels pain from this fixation or from opened skull?

Helps guide surgeons: There are some neurosurgical procedures where an awake patient helps surgeons avoid vital structures that could affect speech/language or other functions which otherwise may become impaired if the patient were not awake. I'm sure a neurosurgical colleague or 2 would like to chime in on this interesting question and give more detailed feedback. Pain is hopefully controlled afterward with medication