She can direct the PM to prorogue parliament. That’s one of her biggest powers. The Supreme Court overturned that, but the power hasn’t gone away. It can be done again and while parliament is prorogued, it can disrupt democracy. Actually, now that I think about it, proroguing parliament should be a dilemma which the UK should have.
In short:
Parliament is being a pain Prime Minister, do you want the Monarch to use the nuclear option?
Pew Pew!