The basic premise is one of the money multiplier. If I give you $100 you weren't expecting to get, then (with a bit of luck!) you will go spend it. 20 other people do the same thing. Let's say that some of them shop at the same place (obviously they do when it's 20mm people so go with me here...) Now, the firm in question feels like things aren't so bad. So they go ahead with some small investment they were planning on making. Other firms do the same. And so it goes.
It's basically to keep the mindset away from saving. Of course the risk (see my luck statement above) is that people take their windfall and save it because they're still worried.
At the end of the day the markets are self fulfilling prophesies. The market is tanking right now because enough people are worried (based on speculation) and are hence selling. That's how it's always worked.
People invest in stocks for 2 reasons: 1) For long term fundamental reasons. 2) Speculation on market volatility for short term gain. 20 years ago, it was mostly (1). Like 95% plus. Speculation was a tiny bubble. Now, it's the other way around. 95% of trading is speculative. The result is more volatility. Over time, prices reflect the fundamental. But that's over multiple years. Short term prices reflect volatility and short-term views.
And don't forget, Apple ALWAYS gives pessimistic guidance on the upcoming quarter...