Drools Guvnor is a centralised repository for Drools Knowledge Bases, with rich web based GUIs, editors, and tools to aid in the management of large numbers of rules. As you know, Drools allows you to create executable knowledge bases. The repository component is where you can store versions of rules, models, functions, processes etc that all relate to these knowledge bases. Access is controlled, and it is possible to lock down access and restrict features so domain experts (non programmers) can view and edit rules without being exposed to all the features at once.
You would want to use Guvnor if
* You have a need to control access to rules and other artifacts
* Have a variety of non programmer users who could use graphical editors to edit rules
* Need to manage versions and changes to the rules over time (and you probably have a lot of rules !).
(Link: Drools Guvnor – JBoss Community)