Serves as a source of technical assistance
Helps you write the best thesis/dissertation you are capable of, given constraints of time, ability, knowledge, etc.
Helps you find resources you need (financial, equipment, psychological support)
Helps you prepare for oral exam and defense
"Keeps the department and other hostile entities off your back"[C]
Introduces you and promotes your work to important people in your field
Gives you advice on the direction of your thesis/dissertation
Gives you advice on career directions
Helps you find a job
After graduation: