I'm not sure on usage of WDL, but the motivating use case make sense to me. Currently there are several ways of creating a 'workflow' (Makefiles, Airflow, Luigi) for things that need to be run over and over again many of those workflows are stored in a database a plain text representation of a workflow is something that could be easily versioned or perhaps even generated by a language of your choice.

A "workflow engine" could encompass the definition of a graph of tasks that need to be run, the monitoring of those tasks, retry logic for failures, alerting for monitoring and some form of UI for viewing progress.