because sometimes an answer from a real human being is better and more useful than from an automated algorithm (like google). Examples are questions where opinions are involved, and those where experience or expertise in the subject is required to get a decent answer.
People use this site to find answers to opinion, subjective, advice based questions. Questions you would ask your family or friends but either don't want to or you did and they don't have an answer. This site is a great place to get people's opinions and experiences, it can be very helpful to people if they use it correctly.