One use case of set actions is to be able to click on a bar and having it drill down to a higher level of granularity.
In this example I will be using sample superstore to create a bar chart of regions against sum(sales) which drills down to regions as you click on the region.
1) We start by dragging our dimension (region) to rows and sum of sales to columns.
2) Right click on the dimension we have on the visualisation (region) and select create> set. We can select any option to add to our set.
3) Create a calculated field:
If [region set] then [state/ province] else "" END
What this is telling us is that if we chose a region in the region set then we will want to drill down into the state/ province which are in the selected state.
4) We now need to add a set action. This is done by going on worksheet> actions> add action> change set value.
We select the sheet we would like to be the action to be applied on, and how we would like the action to be triggered. For this example, we will we want the drill down to occur on selection. We then want to choose how which target set we want. This is going to be the set we used our calculation we created in the previous step. Finally we need to select what happens when run the action and what happens when we click off the selection. In this case we want to assign the value to the set on selection and for the set to be cleared when we click off. We can then click ok.
5) I usually now drag the region set to colour so that the bars which have been drilled down into state/provinces is of a different colour.
This can also be done using parameter actions. In that case we will have an extra calculation after creating a string parameter which states:
[Region parameter[=[Regions].
We will then use a parameter action to trigger the drill down.
My personal preference is to use a set action as there are slightly less options however this is a controversial opinion.