Terry Schurter writes a pretty good article about Business Rules in BPM applications. He talks about how to use business rules, not only for controlling the flow of a process, but also to do smart things.
You can use business rules to transform a typical process into an intelligent process, allowing it to make decisions on his own and reducing costs at the same time in the organization. He shows a good example about a loan application request but you can take this to any level or industry in the marketplace. For example, in a hospital, when a patient comes in and his chart is filled or a prescription is issued, the process can determine internally, base on history, if the patient is allergic, even suggest some kind of disease based on pattern from previous visits.
Possibilities are endless.