You might see it fleetingly in the System Jobs, or less fleetingly if you end up with a "WorkflowExpansion" Failed job in the logs.
The best explanation for what this task is, even 12 years later, seems to be here:
To paraphrase: It's the plug-in that's registered on an entity when you've got one or more asynchronous workflows that are triggered on changes to that entity, and it queries to find out which workflows should actually be run and runs them.
(I'm only looking because we've got some failed entries in the logs. I guess that means that any workflows that should have been run for the related entity based on that update.... haven't been.)
