Setting the trigger node of a workflow to "Triggered by a Workflow" allows you to split workflows into smaller and more understandable workflows, create portions of workflows that could be used in multiple workflows, or remove users from a workflow if they don't meet certain search criteria.
Creating a Workflow to be Triggered by Others
Using the trigger node "Triggered by a Workflow" will allow a workflow to be triggered from any other workflow.
From that point onward, all other features, such as action nodes and filter nodes, are available for use.
Triggering a Workflow from Other Workflows
Using the "Trigger Workflow" action in an Action Node will allow you to trigger any workflow that has it's trigger set to "Triggered by a Workflow."
At that point, the user will be added into the other workflow, while also continuing down the original path in this workflow.