Set analysis is very powerful.
Set Analysis is a great feature in QlikView that lets you, as a developer, take control over what your charts display and allows calculations that wouldn't be possible otherwise, at least not as dynamically. To understand its inner workings, we can compare it to how selections that are made using listboxes work.
With UI selections, we can say that whatever is selected affects the entire document, and all of the charts only display information associated with the set of data corresponding to those selections; plain and simple. In a way, that is basically what Set Analysis does. It restricts, predefines, or extends the set of data that charts base their calculations on. Using a set expression, we can, for instance, specify that a certain chart should perform an aggregation only based on records that meet a set of criteria in certain fields (for example, Region A and Region B from the Region field), even if the non-matching values are part of the user's selected record set. We can also use Set Analysis to expand the selections made by the user to show, for example, results of the previous year even when it has not been actually selected. For example, when the user selects the year 2012 and a table displays data for 2012 in one column and 2011 in the other.
Hope this helps