Why is this tab actually really insightful, you may ask?
Well, with Devices if here are more viewing on a Mobile device, you may want to consider having more 1 Container, Structures. With Videos, Buttons and clear visual clickable content within your email. Whereas for those viewing on a Desktop, you may want to use 2+ Container, Structures within your comms because you have more display on a computer monitor to view content.
Mail Apps. Something that may not appear as insightful, but is actually incredibly helpful and intuitive. By seeing if people are using iPhones compared to Android, you're able to consider resolution and sizing of screens. For example with iPhone X2 all the way to iPhone 11 Pro Max the average screen size was between 5.8 and 6.5 inches. Whereas Android devices vary from 3 to 6 inches. (Data from https://twinr.dev/blogs/a-complete-guide-to-android-screen-resolutions-and-sizes/#:~:text=The%20size%20of%20an%20Android,necessarily%20mean%20a%20higher%20resolution.)
The 'Device' tab will help to provide insightful data to influence your design choice of templates, layout, style and more.