The main sewer line of your plumbing system is just one component of the overall system - but, it is a very important one.
In fact, you could argue that it is the most important part of the system to keep professionally serviced - due to the sensitive nature of the materials in the main sewer line.
When you brush your teeth, flush the toilet, or hear the dishwasher drain - all of the waste and used water goes into the drains the respective plumbing fixture. However, once the drain has captured the material - it then drops the material into the main sewer line.
And, since the main sewer line runs under the ground outside of your home - you can imagine the mess that you would have if the line cracked and began to leak.