Homes are built so that all of the drains flow downhill to the main sewer line. However when you have a basement it is often that this is below the main sewer line. Any waste from below the main sewer line level can’t use gravity to flow out of the house. In this case you need an ejector pump to push the waste from below the sewer line to above and out of the home. This is similar to a sump pump which only pumps overflow water.