For keeping the power plug adapter in place. For us Europeans using an adapter, having bought a Google Home before the European launch, and not having American or British power outlets (type A B or G). Plug C, E and F FTW! :D
This thing is made for an adapter we already had lying around, it's 29,5 by 12,5 mm, but will probably work with smaller adapters too. If you want a version that uses the adapter you have at hand you can always ask, and we might make one for you.
The Google Home mini, max and hub are using another kind of wall plug/PSU.