Slug catchers play a vital role in natural gas pipelines; these systems often contain significant amounts of liquids that can damage related equipment. A slug catcher is designed to allow expansion of the gas stream, slowing the velocity and providing a settling area for the liquids.
In gas gathering pipelines, water and condensate will gather at certain points along the pipeline.  The liquid in the pipeline causes a pressure drop which is undesirable.  Pigging is used to clear all the liquid from the pipeline which results in a large volume of liquid that needs to be handled before the gas reaches the gas transmission plant.  This large volume of liquid is handled by a slug catcher.