The rule engine will read all files of a project and use them to apply the rules against a set of data. this way one can check data or use the rules for filtering the data.
E.g. in an ETL process the rules may be used to extract or process only data that is according to the rules. At the same time one can output the data that does not fit to the rules. This helps to determine why some data passed and other data failed.
For interfaces to other systems the rule engine can be helpful to have some user define, what data should go into the interface file - based on certain rules.
Again - the advantage is that the rules and logic is define outside the ETL process, outside your application or outside your website and can be maintained in a certral place. The maintenance can be done by users and - when rules change - one won't have to change the code.