For example, you create a Service Desk screen for a new Workflow Template and create rules to dynamically show hide aspects of the screen. Those rules should also be available for the Portal Request Submission screen of the same screen set. So that similar behaviour can be set up easily and efficiently.
Logical suggestion. Just as an fyi, if you don't mind getting your hands dirty you can achieve this via SQL scripting in SU_SCREN_FIELD_RULE - just duplicate NAME, ENTITY_TYPE, XML_RULE, FORM_REF and STATUS with a unique REF, and set SCREEN_TYPE to 1 for Details screen or 2 for Submission screen. Don't forget to update NUMBER_ALLOC in SU_NUMBER_ALLOC for the new REF.
If this doesn't make sense then don't do it!! Or at least do it in dev first :-)