2D Layer View allows you to asses the individual layers of a print after configuring the slice settings outside of the Preview Window.
To enable the 2D layer view, open up the preview window and select Layer View.
You will see the basic movement controls above, but not much else. Click Generate to show the rest of the features.
This will allow you to use the slider on the right hand side to view the individual layers as they have been sliced, as well as seeing the print time, print volume, and total weight of material used.