You need determinism in all multiplayer games that do not have an authoritative server. Although I would argue that you would want determinism in all games. In

You need determinism in all multiplayer games that do not have an authoritative server. Although I would argue that you would want determinism in all games. In terms of this game in particular, it's a multiplayer coop action shooter so you want everything the behave the same way on both players machines. The player models should be in consistent places in the world, bullets should fly in the same direction on both, grenades and other physics objects should bounce the same way. If these things do not happen then you will have diverging realities on each of the machines.