This only matters for things like encryption and secrets. Imagine if you published your game online as multi-player, someone could devise the randomness and predict the next moves in the game, which could allow them to cheat. For encryption, a predictable RNG can break the entire scheme.
In your casual sense of just a single-player game, there's no harm, but the idea in replacing MT with something like PCG is to be more secure/safe by default, since you never know what someone will use rand() for.
In your casual sense of just a single-player game, there's no harm, but the idea in replacing MT with something like PCG is to be more secure/safe by default, since you never know what someone will use rand() for.