You use set when you are creating an instance of an object.
The variable there becomes a instance of an object in your case it is ActiveDocument.PrintReportEx("RP01")
While creating the instance you have passed the value RP01 which is the ID of the Report.
Objects have properties, methods and events
Next you are using printResponse.URL
URL is a property of an object here and you are reading the value of the property.
On the other hand when you do not use set its just a simplae variable to store the value.
Hope this helps.