If the Play Store already packs the OBB file with the APK, and puts it in the OBB folder, then all I have to do is unpack and copy the files to Internal Storage.
Why would I need to use DownloaderService? It seems like a lot of overhead and complicated code if I can accomplish what I want without it. What would be the advantage of using DownloaderService?
Another question: if my APK upload size is 5 MB, but when I download it from the Play Store the download size is 95 MB. Why is it including the OBB file in the download from the Play Store?
You are absolutely right, in theory you don't need the downloader service if everything is working perfectly. However users often decide to "clean up" their device, either manually, or using so called cleaner apps, most of which don't clean up anything properly. These apps could delete your OBB files, or the user could, even when they shouldn't.
By building the downloader service into your app you can recover the OBBs, even if the user makes a mistake like this, then your app will keep working.