Some form or other of pain killing or anaesthesia is a modern requirement for most forms of surgery. There are a large number of agents and methods to achieve the same goal: avoiding discomfort and at the same time ensuring maximal safety. All of these carry there specific pro's and cons. Different surgeons prefer different anesthetic systems. Also, the patients tend to have preferences, oftentimes based on previous experience. Where anaesthetists are involved, a lot depends on their attitude and experience.
The patient has also her say in which method will be used. Some have had bad experiences with general anesthetics in the past and are looking for a lighter alternative, whereas others cannot bear the thought of conciousness and want to be "out" for the entire surgical process.