The logic is to fire an event. Without an event we cannot store value to a variable. From the first page u clicked a value which will be passed to the prompt as it is referring to the same column. Now we need to fetch the value to the presentation variable because we need the value in another subject area/column. So to hold the value in presentation variable we need an event. For that we are making a hidden prompt with auto apply. So when you pass the value to prompt and open the new page , value will be assigned to presentation variable.