Okay, lets say that in your paticular scenario there was a misplay, you saw your opponent forgot to carry out a mandatory effect (it's a regonal tournament in my hypotetical scenario) and nothing more has happened than a phase change. Simple right, you just back it up to the previous phase, the mandatory effect is applied, move on from there....no harm, no foul.....however, perhaps this is the 5th time in the last 3 matches that paticular player has attempted to not follow through with a mandatory effect this tournament, you don't know the contents of his / her hand do you? Was there a motive than was less than honorable involved? Was it intentional? You're assuming that it isn't (I tend to be overly trusting as well mind you)
Such things judges need to be alerted to. If this player has often "forgotten" to carry out a mandatory effect the judges want to issue the warning so that when it happens again we see a pattern developing and can keep an eye on this person for the rest of the tournament. Perhaps it's just forgetfulness, fair enough, we'll try and keep an eye on this person to remind them so it DOESN'T get to an irrepairable game state in the future, if it looks as though he's trying to get away with something (unbenounced to you) we'll do what we can to catch them in the act.
What may SEEM harmless isn't necessarily so. There may be more to the story than you realize. That's what I was talking about in my initial reply....the detective work that you as a player aren't allowed to do, haven't the resources, time, or information to do.