Most often people will want to serialize a RuleBase (the binary rule base object you get from PackageBuilder). This is a handy way to deploy rules in binary form. All aspects of binary rules are serializable.
Othertimes, people may want to serialize a Working Memory instance with pre-populated facts for quick loading later on (as they may have standard pre conditions). Serializing the working memory can save time as you don't have to assert all the objects again. Of course, all your objects have to be serializable, otherwise you can try one of the alternative serialisation technologies mentioned above.