Becoming a more serious runner means that you become seriously more intense about data. You want to know exactly what your times are, what they mean, how you can do better.
What this means in a big race like the NYC Marathon is that there are 48,000 runners who ALL want that data. And maybe their families want that info too.
So as you can imagine, the logistics of timing a race like this is INSANE. Check out how it gets done and done accurately.