A product owner that has screen mockups to work with has a powerful tool to elaborate user stories. Pictures enhance collaboration and understanding by giving the software development team an alternate method of receiving software product requirements from the product owner.
From a QA perspective, the mockups serve as a guide for testing the user interface to ensure that the user interface is developed as requested by the product owner.
It gets tricky though when the user interface mockup illustrates product functionality covered by several user stories. The one easy way around this is to just add the user story number to the mockup and use themes.
