In simplest terms, a sump pit is a gravel filled pit finished with a grade level, pressure release overflow cap.  If the pit would ever reach capacity, the cap would lift and release the excessive water gradually over the area.  Whether it is required by your local municipality, or you simply have no other practical means to discharge water, a sump pit may be necessary.  The purpose of a sump pit is to create an underground retention area, where water is directed and held, until it is able to soak back into the soil naturally.  Sump pits are typically used for the discharge of downspouts, sump pumps or even yard drainage.  The size of the pit can vary depending on municipality requirement and/or the volume of water it needs to absorb.