Very often, when stocks are moving directionally, there is a broad theme that is driving the day's trade. Sometimes the theme will be evident in the relative movement of sectors. For example, themes of feared economic weakness might lead defensive sectors (such as consumer staples) and large cap, blue chip stocks to outperform growth sectors (such as consumer discretionary) and small cap, speculative stocks.
Other times we see themes illustrated in intermarket trade. Fears of Fed tightening might lead stocks to fall, Treasury yields to rise, yield curves to flatten, the U.S. dollar to strengthen, and stocks to fall.
When we identify a theme, we're then in a good place to observe the sectors and asset classes that are participate and see which ones seem to be leading market moves. We can also determine if themes are continuing as the day progresses, or whether there are shifts in the themes. Many times these shifts will first occur away from the stocks and stock indexes you are trading, providing a useful heads up.
Understanding why markets are moving can be very helpful in anticipating where they might move going forward. Much that seems random makes sense once you look at the right things on the screen.