I have read many software specifications over the years that simplistically list being “user friendly” as a design requirement. User Experience (UX) designers can get a chuckle from this, knowing that there is a fair bit of effort involved during the design phase to make that happen. The seventh rule of Shneiderman’s Eight Golden Rules talks about supporting an internal locus of control, and may be the closest thing to a simple description of user friendly of any of the rules.
A description of the notions of internal vs external locus of control can be found here. Simply put, it boils down to whether you view yourself in control of your own fate, or at the mercy of external forces. For example, if you get a promotion at work, is it because all your effort is being noticed and appreciated (internal locus of control), or is it because you feel you just happened to be in the right place at the right time (external locus of control).
When we apply this notion to software design, we want to make sure the user feels in control of the software and confident in how to accomplish their tasks. A user should never be wondering “How did I get to this screen?” or “What do I need to press to do my task?” Navigation and task activation should always be clear and well-marked.
Navigation in particular often suffers when software isn’t well designed. Things as simple as adding an extra keystroke or button press can make a great difference when looking at tasks that are life critical or highly repetitive.
There is an often referenced study (Mayhew and Bias, 1994) that describes how AT&T redesigned a system in their call center to simplify their workflow to not only make their worker more efficient but saved them hundreds of thousands of dollars in training expenses.
Keeping the user in control makes them comfortable and means they are more likely to continue using the software in question, possibly even recommend it to others. Following this particular Golden Rule is not only good design practice, it's good for business as well.
Study.com website, last accessed April 15, 2016, http://study.com/academy/lesson/locus-of-control-definition-and-examples-of-internal-and-external.html