Drools Guvnor is used if you have the following requirement
a) You need to manage versions/deployment of rules.This is a scenario where you have a huge number of business rules and they need to be managed using versioning.
b) You have multiple users at different skill levels and they will access and edit rules.
c) You have lot of business rules and they are growing.And you do not have any existing infrastructure for rules management.
Guvnor can be used as stand alone or integrated with IDE.