For woodturners, a finial is used to decorate the top or end of some objects. In architecture, finials are used as a decorative feature, to mark the apex of a tower, roof, or gable. Roof gable decorations are especially popular. Wooden finials are also used in furniture design as a decorative element and in period style/heritage buildings. Also, Capitols, used as post tops on period fencing, staircase Newel posts Entrance gates, etc.
Post capitols add some artistry to either plain old square or Pyramid topped posts. These come in your standard Windsor, Ball and Acorns to exotic designs from yesteryear. As well as being decorative, finials and post caps are vital to the health of your deck and fence posts.