This file apparently controls/manages what apps and files are believed to be on the phone. Here is why I think that.
1. The galaxy Prime (don't laugh) was constantly "full" where it couldn't update anything, couldnt accept texts, etc.. on and on.
Attempting to Uninstall Facebook? done daily, only to look again and see it still installed.
Try to delete or move Pictures? nope - "Failed to move" "Failed to delete" file. Pics and Videos remain no matter what you do.
Final attempt? "Clear data" on the Media Storage file and Boom!!
Finally, the pictures, videos, we had tried to move or delete, and Facebook uninstall FINALLY ARE REMOVED!!
And space is back, texts are flowing, no more complaining of space etc..
I am guessing this file gets to size and point where it cannot be efficiently managed by the phone/android.