Rules serves the need for automating processes like comment moderation, customizable e-mail notifications or event-based calculations on social platforms. These are built by using generic tools like Flag, Organic groups and message module. More specific solutions like Privatemsg, Feeds, Activity, User point or Voting rules also tightly integrate into the system.
Interacting with external web services is another field where configurable automation is key to success. Tools like Web service client module or fluxkraft allow integrating services like twitter and facebook with custom workflows on any Drupal site.
Besides that, Rules is also used for specifying optimized cache invalidation workflows using the the Cache Actions module or to help out with Multilingual workflows with the Translation Management Tools or Lingotek integrations.