Often times in film, you will have several people working on different facets of a scene at the same time.
Sequencer supports this type of workflow through the Subscenes Track which allows you to layer Level Sequences on top of each other to produce a final sequence.
You may have an animator creating the animations for the sequence, while someone else works on the camera and another works on effects which are all part of the same sequence.
Doing so this way allows people to have their own working Level Sequence and work independently without altering content in other scenes.
In this example we produce a small scene using the Subscenes Track & Compositing of multiple Level Sequences.