Generally speaking, a rule repository is like a souped-up source code control system. In fact, rule-based systems that don't include a rule repository of their own -- including OPSJ, Jess, and JBoss Rules -- typically integrate with a source code control system such as CVS or IBM Rational ClearCase to bring version tracking and access control to the rulebase, as well as let auditors see who made what changes to the rules, when they made them, and why (or at least the reason they gave).
A CVS will likely meet the needs of most projects where the number of lines of code and number of developers is relatively small. However, as the numbers of rules and project participants increase, the more a full-featured rule repository can help. The rule repositories in the two leading enterprise BRMS, Fair Isaac's Blaze Advisor and ILOG's JRules, go far beyond a CVS to provide:
Really, really fine-grained access management;
Really, really fine-grained change tracking;
Search for objects accessed during the last run of the rules;
Search for objects not touched by the last run of the rules;
Search for which rule fired the most on the last run;
Search for which rule fired the most on the last 10 or 20 runs;
Search for which rule never fired on the last run;
Search for which rule never fired on the last 10 or 20 runs;
Search for which rule has never fired;
Search for which rules always fire and how many times, on average, for each;
Stepwise debugging for each line in the rule (a lot of overhead for this one).
In short, a rule repository offers extensive change auditing (essential for compliance), and, even more important, deep reporting and rule analysis that supports debugging of the system when something doesn’t work according to plan, and optimization of the system when something doesn't execute as fast as you'd like. It can be a key ingredient to bringing business rule management to the enterprise.