From a high level, here are the 3 reasons I have decided to learn Groovy:
1) Capabilities - It supports dynamic concepts and features that are not available in Java but are available to Ruby, Python and JavaScript developers. Overall it reduces the amount of boiler plate you have to write.
2) Progression - It is a super set of Java, therefore it has that feeling of natural progressions and familiarity that we got when moving form C to C++ and then from C++ to Java
3) Productivity - It facilitates Agile best practices such as
unit testing and ability to more easily mock objects
Regards
Paul