Summary: People have very limited ability to keep information in their working memory while performing tasks, so user interfaces should be designed accordingly: to minimize memory load. One way of doing so is to offload items to external memory by showing them on the screen.
Human working memory holds information relevant to the current task; when tasks are too hard, users should be able to offload some of the working-memory burden to user-interface features that can serve as an external memory.
Raluca Budiu is Director of Research at Nielsen Norman Group, where she consults for clients from a variety of industries and presents tutorials on mobile usability, designing interfaces for multiple devices, quantitative usability methods, cognitive psychology for designers, and principles of human-computer interaction. She also serves as editor for the articles published on NNgroup.com. Raluca coauthored the NN/g reports on tablet usability, mobile usability, iPad usability, and the usability of children's websites, as well as the book Mobile Usability. She holds a Ph.D. from Carnegie Mellon University.