Why would you design the inside of a package and fill it with content?
Perhaps it’s because it’s cheap to do. Perhaps it’s because you can. Perhaps it’s because you want to signal status and quality.
Or perhaps it’s because you know your customers do recycle, they flatten the package to save room in the recycling container, and you want to connect with them one last time before they put the package away.
In any case, it can be beautiful.