Only() function is like other aggregation function, aggregate the values provided it in a particular field and give you one value. It is very simple function and that's why it becomes sometimes difficult to understand why we really need to use it.
In this video, I've give to practical use cases in which I've utilized the only function and hope it will give you good clarity in terms of its practical usage.
Hope you'll find this video useful and an addition to your learning of QlikView.
-Abhishek
QlikView Tutorials | QlikView Expressions | QlikView Only Function Explanation and Application