Possible use cases are the following:
For example, we have to execute graph B, only if another graph A finished without any error.
So there is some kind of relation between these graphs.
We can achieve this behaviour by creating a graph event listener.
We create a listener for graph finished OK event of graph A
and choose an execute graph task type with graph B specified for execution.
If we create another listener for graph B with execute graph task with graph C specified,
it will work as a chain of graphs.

Figure 24.3. Event source graph isn't specified, thus listener works for all graphs in specified sandbox

Figure 24.4. Web GUI - e-mail notification about graph failure

Figure 24.5. Web GUI - email notification about graph success

Figure 24.6. Web GUI - backup of data processed by graph