Because earnings drops are causing stock drops and stock drops are causing investors to pull out capital, etc. And the earnings drops and stock drops are going to exaggerate the unemployment rate, by causing it to spike higher and one of their mandates covers employment.
So, while they won’t be able to reverse a market downtrend (because they’ve never been able to), they can potentially stem how deep the recession goes and how long the bear market lasts, had they not done anything.