A rule engine may be thought of as a more advanced version of an if/then statement interpreter. Rules are the if/then phrases that are interpreted.
Advantages of Rule Engines:
Greater flexibility: keeping your rules into a Knowledge base lets you adapt easily to your decisions when they are changing.
Easier to grasp: Rules are easier to understand than procedural code so they can be effectively used to bridge the gap between business analysts and developers.
Reduced complexity: When embedding lots of decision points to your procedural codes it can easily turn your application into a nightmare. On the other hand, rules can handle much better increasing complexity because they use a consistent representation of business rules.
Reusability: By keeping rules are kept in one place leads to greater reusability of your business rules. Also, traditional procedural codes often imposes unnecessary variations of base rules which are therefore more difficult to reuse in other contexts.