It's not necessary per se (much less catastrophic if you do otherwise), but it's certainly much more convenient. It's only necessary if you rely on simulations

It's not necessary per se (much less catastrophic if you do otherwise), but it's certainly much more convenient. It's only necessary if you rely on simulations being identical, which would happen if you are using deterministic dead reckoning or lockstep networking. Client side prediction and server side cheat detection are normally not so strict.