I find it necessary for a fighting game like Injustice 2, where there aren't rounds but instead each fighter gets 2 health bars. The clash function allows you to try even the field a bit after losing your first health bar. It is very costly too, you only get one per match and to really regain much health with it you need to spend a good portion of your super meter. Even if you spend no super meter than at best you just break the opponent's combo, or you could take decent damage if the opponent spends their meter. Also like any "cheap" thing in a game, especially a fighting game, nothing is stopping you from using it yourself.
From less of a gameplay mechanic point of view clash also works to help bring the comic book style battles to life in the game. Banter between comic book characters is classic and clash allows for them to naturally integrate that into the battle, and make it a focus rather than just having the characters banter over the sound effects as they fight.