Financial crises typically induce a large and sudden fall in aggregate demand, owing to negative wealth effects, a collapse of confidence or, more generally, both. While these shocks are difficult to quantify in real time, conceptually they are similar to the shocks addressed by monetary policy on a continuing basis.
To simplify, downward pressure on price developments associated with a fall of economic activity can be countered using standard interest rate policy, reducing official rates so as to spur spending and bolster confidence. Such measures serve the maintenance of price stability. The timing of the interest rate response is crucially important. Financial crises can strike suddenly.