Using a Triggered by a Workflow node 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 a Triggered by a Workflow trigger node allows 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 its 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.