Showcase represents a UI playground where theme can be combined with layouts, container components, and individual components to illustrate a layout concepts outside of the application. They are often a simplified version of the view, as the data that is loaded is mock and the controllers are often very "dumb".
For Developers
When to use Showcase
Showcase should be used when you need to rapidly prototype a portion of a view to check for consistency, acceptance tests, and/or design QA before or during implementation of the feature in an application module.
Showcase often combines many components and classes together in a specific way that is useful at the time. That being said, as the production application view deviates, Showcase often becomes archaic. It's best to create Showcase then only in the cases where 1. it will speed up development and design QA, even if that means deleting the Showcase once the story is complete, or 2. it serves a specific long-term purpose to show how something should be built across application modules (side navigation is an example of this). If what you need to build is in neither of these categories, chances are Showcase is not the right solution to your situation.
For Product Owners and Designers
One could look at Showcase like looking at a portion of a prefabricated house. You may see some wires and the inside of a couple walls, but you are there to review a specific set of components working together. Be sure to ask questions and have the developer document what the showcase is (and is not!) meant to be an example of.