Replay system
Plays a match back afterwards by storing the players' inputs and re-running the simulation, not by filming the screen.
A deterministic engine need only store the start state and each player's button presses per tick; feed the same inputs through the same simulation and you get exactly the same match again, at a fraction of a video file's size. Doom and Quake saved such demos back in the 1990s, and the technique drives killcams, esports replays and cheat review today.
The price is that everything affecting the outcome must be fully deterministic — the same floating-point result on every machine — or the playback drifts apart (desync). The alternative, saving the full state every frame, is simple but too heavy for longer matches.