Well, this question is asked by clients, frequently.
The attached Case Study will address - "How does UI work".
A nice and huge software was made by a Bangalore based Corporate, for telecom operators. The main function is to "block" and "allow" predefined numbers with respect to predefined time and location. It was a marvel, but very complicated also.
A meeting was going on with a Kuwait based telecom giant. They were interested in the product, but were at a loss in understanding the functionalities. These functionalities were designed in such a way that if the back end data is uploaded properly, the front end will work fine. But the design flaw was, the back end and front end was mixed together, and any user, at a first look, will be at a loss with so many options. We were called at this stage.
We studied the structure and understood the complexities. Pointed to some overlaps, logical incompleteness etc. But, most importantly, we "divided" the software in two parts. One part for the "assistant" who will upload the phone numbers, make the groups etc. And the other part is for the "boss" who will click on a button to start or end a predefined task.
We started calliing it "one touch screen".
And it took a couple of meetings after that to start ending the deal :)