Not closing your damper is comparable to leaving a window in your home open during the winter. Over time, California homes experience earthquakes, settling of the ground they are built on as well as any number of other maladies, which causes the damper inside the chimney not to close properly, which means it is not sealing the opening completely and has become less effective in keeping your HVAC treated air contained inside your home.
A top closing damper will seal the chimney from the top which closes out rainwater, snow and dew as well as all of the small animals that enjoy dwelling in a chimney as well the smallest of insects (bees). PLUS, a top closing damper will keep your air-conditioned air from escaping your home in the summer months and your centrally heated air from escaping during the winter when you are not burning a fire. The energy cost savings over time more than pays for the top closing damper.