Build your own vs. built-in downloads

RN Newbie question here: I need reliable downloads that should complete no matter what the user does with the app (run in the background, resume etc.), and I saw that there are a number of implementations out there, including this package.

I'm already using RN-FS for local file system stuff, but can't help but wonder why there are so many packages building proprietary download functionality, when the both IOS and Android support native download capabilities that bring all that already to the table (e.g. https://developer.android.com/reference/android/app/DownloadManager). Are there reasons why we shouldn't be using those?