Just a quick glance at an online stock ticker gives me a wealth of information. Not only do I get an instant idea of how the overall market is performing, but I also know what the specific stocks I am watching are doing.
I also have stock symbols on my ticker that represent certain sectors such as gold, oil or semi conductors in the technology industry. Because I am involved in short term trading, this is vital to me on a daily basis.
The Dow, Nasdaq and S & P will often move together, but that is not always the case. Anytime I see any big difference in them, I always take a closer look to see what is going on.