So that's why. Hope something like "Set as Default Cover" per game is implemented. Most of the time, the automatic default covers are correct but there could be 1 or 2 that misses so we could just manually assign those.
I actually did this just now. For me, I set the Fanart - Box - Front to Box - Front because the fanart was more uniform. This is specially true for arcade games where there isn't really an official cover art so fanmade media looks better. Or sometimes, the automatic scraper misses a game or 2, and you can just manually update those instead of (re)importing an image pack.