Some people will argue that user interface mockups smell of big planning and have no place in agile, I disagree. Let’s face it; it’s hard for a product owner to communicate to the software development team how a software product should look and work without pictures.
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.