Philosophy is indeed a public service, even if it's rather a niche one. People have a large access to many kind of arts and activities. Some will enjoy reading etiology books or discussing ethical themes, some will visit museums, some will watch Scorcese's movies, many will watch Avenger entertainments. We all need some kind of artistic culture around us. So philosophy is a public service because some part of the public enjoys it to the point that it is important in their life.
This "public service" status is not restricted to arts and entertainment. For instance, the science on the human evolution has no practical goal — extending knowledge has no direct impact on us. Just like philosophy, it will not really influence the way we live. Yet many people want to know more about human origins, which is a excellent reason for continuing research.