The ability to layout a map, for example one that uses half the sheet of paper and duplicate it exactly, layers, symbology, extents, etc. Or for the data frame to be exported as an image and automatically placed next to the existing data frame as many times as needed. This could be handy for small handout maps.
If the entire layout was duplicated, data frame and layout so you could change just one layer and display them side by side. Before and after or map with a different basemap or a different series of layers of interest.