I want to know what the purpose of the system app "Media Storage" and the media scanner is for and why for any reason a user would choose to not scan media on boot.
Media Storage, package name
com.android.providers.media, is an implementation of MediaStore:
The Media provider contains meta data for all available media on both internal and external storage devices.
It scans and stores media file info for quick access, and provides secure (
content:/// scheme, as with all other providers) URIs pointing to the files for access by other apps.
By default, Media Storage has a broadcast receiver
MediaScannerReceiver that listens for
android.intent.action.BOOT_COMPLETED, so media scanner will launch on boot. Such receivers are unlikely to be disabled by end users unless with root permissions.