Where is the Java scripting engine used? [closed]

Question

How do I effectively use the "Scripting Engine" inside Java?

What are all the right use-cases to use the scripting engine?

Is there an open source project using "Scripting Engine"?

One thing comes to mind is "Closure, Functional programming" support is possible, but it is more of technical use than "Application Requirement".

Configurable plugins are OK. But still so many patterns (visitor, decorator) on high level can do the same.

I don't know the requirement well... how effectively it could be used in Java EE patterns... where it could complement with the existing patterns.

Moreover I would like to see more answers with some business usecases. Maybe like finding a complex discount for a product during sale based on membership or location. Finding ranking for a complex algorithm. Especially why not Java in some scenario? (or C# in .NET world)