Request type rules are a powerful way of configuring complex interactions between fields on a request. You can use them to set up automatic population of request fields, change field attributes, or otherwise affect request field behavior based on various dependencies.
One of the most common rule configurations is to set the default workflow when a user creates a new request. From a practical standpoint, you typically known which workflow is appropriate for a request of a given type, and you do not want to offer the user a choice when he creates a request. So, the Workflow field is typically disabled, and a rule is defined to automatically set it to the appropriate value.
Keep in mind that there is some overlap between request type rule functionality and status dependency functionality. It is extremely important that you understand how status dependencies and request type rules can interact (and possibly produce unintended results), and plan carefully before you implement either. We strongly recommend that, before you configure request type rules, you thoroughly review Configuring Request Field Status Dependencies, and be familiar with all the way in which rules can affect system behavior. Also see Important Considerations for Configuring Advanced UI Rules.