Thanks for posting on PSC.

A workgroup is defined as part of the organizational structure and is used when configuring a Workbasket. 

A workbasket is a named queue of open assignments that are not associated with an operator. A workbasket is defined by an instance of the Data-Admin-WorkBasket class.

A workgroup is an instance of the Data-Admin-WorkGroup class. A workgroup can identify a user who is a supervisor, and a set of workers and workbaskets that report to that supervisor.

Now, let's come to your question. For Ex- An operator could possess multiple skills or belong to multiple departments and called on the ad-hoc basis or any other circumstances. Multiple Workgroup ensures the operator to belong multiple workbaskets across the departments/units within the organisation thus enabling an operator to work on the work-objects out of their default work-basket. 

- Concept behind workgroup/workbasket.

- Scenario

Hope it answers your query, kindly notify by marking this post as answered for larger Pega audience.