Preview Mode was added to InterSystems IRIS Business Intelligence to give designers a quick view of what their resulting Pivot Table will look like without needing to wait for the results to fully execute. This can be beneficial when designing pivot tables because if you are dragging and dropping elements to see how they look/work in your pivot table and seeing if they have the desired data. Since you are exploring and designing, you don't necessarily care about the results at the moment, but you would still like to see how your table looks with the changes you have made.