When mass deleting pokemon, (one at a time), you might not always look at the names of the pokemon. So you might happen to accidentally trash a pokemon you did not want to trash. Favoriting a pokemon would prevent it from being able to trash it, whilst (maybe) not notifying you when you put it in your first 6 slots. This way you would not be able to trash your (only) HA or 5IV that took you so long to find/breed/save up money for!
When mass deleting pokemon, (one at a time), you might not always look at the names of the pokemon. So you might happen to accidentally trash a pokemon you did not want to trash. Favoriting a pokemon would prevent it from being able to trash it, whilst (maybe) not notifying you when you put it in your first 6 slots. This way you would not be able to trash your (only) HA or 5IV that took you so long to find/breed/save up money for!