Usually traders like to buy when prices are going up and sell while they drop-- it's the old "trend-is-your-friend" (AKA "don't fight the tape") strategy. The exception is dollar cost averaging, where a saver just plunks money at set intervals into a fund and forgets about it until say, 10 years later.
Both methods work because they're suited for different kinds of investing goals.
I've concluded there's no trend in this market. Lots of negativity today, but look closely and you'll see just bloated corpses of dying bank stocks. All the good stocks I want (INTC, MSFT, miners, energy) are either up or have not dropped to my buy targets. I think if I waited for those to rise, that would be too late. Similarly waiting for them to peak and fall would be tricky. So in the zig zag market I am trying buy and sell targets instead.