Instead of assuming a return rate, a simulator replays what a real stock or index actually did, day by day, over whatever date range you pick.
A calculator projects forward from a rate you assume, the same smooth curve every time. A simulator tests a specific behavior, like investing a lump sum, spreading it out monthly, or only buying on dips, against a stock or index's actual daily closing prices, so the outcome changes with genuine market volatility instead of following an assumed curve.