The documentation mentions that there is an activity extension that allows you to specify an expression in your task-definition to set the initial due date of a task when it is created. The example given is,
When specified here, is this for informational purposes only? I know I can query based on due dates. But does the engine do anything with this dueDate value?
Due date is just task attribute. As you mentioned, there is possibility to make a query based on the due date, but engine does not trigger any event in the case when due date is reached.
If you want to trigger any further execution use e.g. timer boundary event associated with the user task.