Tackle “from behind”

I think one of the reasons that getting the ball can have an influence is that sometimes (though not always) it can be used as an argument that the player was not being as careless (or reckless, or excessively forceful) as if they had just clattered the opponent without even getting close to the ball.
Getting the ball first still is important though - because incidental contact after proportionate force is used is still perfectly fine. So if the ball was hit first it still matters, but doesn't necessarily mean no foul - and to be honest, it never has.