Why do most city codes require a trench dam in gravity sewer line from house to the main sanitary sewer - what does this dam actually do?

Why do most city codes require a trench dam in gravity sewer line from house to the main sanitary sewer - what does this dam actually do?

Trench dams are there to prevent ground water from running under the sewer pipe and eroding the bedding gravel.
The dam may be built out of clay or it can be manufactured, To learn more click on: Features (http://trenchdam.com/mpc/docs/Site/Features.html)
Regards, Tom