It prevents a dictator by acting as a “constitutional longstop”
The idea that the Queen can act as a check on politicians may be comforting, but it's a fantasy. The opposite is true - the monarchy gives huge power to politicians, while the Queen just does what she's told. That’s why republicans want a head of state who is both elected and effective, someone independent of politicians who can act as an impartial referee of the political system.
There is simply no constitutional mechanism by which the Queen could stop a potential dictator, other than by becoming one herself. The greatest safeguard against tyranny is a culture of active citizenship and a political system with clear rules based on equal rights, transparency and accountability – values to which monarchy is directly opposed.