What is an Engine?

Engine is just a small application which can run within other Application. It has its own routes, assets, controllers, models, helpers, Views and even Gems

Bcui Main Application is also a kind of engine but with some added features. 

When would you need one?

Ideal case for engine is when you have a completely isolated domain from the main app. It has its own controllers, assets and models which doesn't share with rest of the App.