The double panel map is perfect for comparing datasets collected at different times. Here, we are showing the traffic congestion in Barcelona during the Mobile World Conference and then the traffic one week later.
The double panel is both useful for when you have obvious differences that you want to show immediately and for cases where you want to encourage the user to explore the map carefully to discover the differences. In the case above, it may not look different immediately, but if you were planning a drive during the MWC, you may want to look more carefully.