Each day, the members of your household use the various parts of your plumbing system to get chores and tasks done.
Things like cleaning up dishes after dinner, taking a shower to get ready for school and washing a load of clothes to have clean uniforms for the soccer tournament are all tasks that need to get done for your family to have the quality of life you enjoy.
Unfortunately, these tasks do not come without some kind of waste product. Be it used water or particles of solid organic waste, using any one of your plumbing fixtures or appliances results in some kind of by-product.
Your sewer system is responsible for managing all of this material. While the drains inside of your home are the internal sewer system, the main sewer line that is buried under your home and all of the pipes that it attaches to are your external sewer system.