Back in the day, when the green flag flew at the start of a race, many times the field would fan out with everyone trying to gain some type of advantage under acceleration. I remember old footage of the Firehawk series where two neat lines of cars would turn into mass mayhem at the drop of the green and there would be eight cars abreast going into a corner with enough room for one line of cars! The resulting chaos would end up destroying some perfectly good race cars, cost team owners a ton of cash and generally ignite a firestorm of "who done it" accusations up and down pit lane. On top of that, the race would have to be restarted, and the whole thing would be a waste of everyone's time.
To make things a bit easier on owner's wallets and to help give the fans an actual race to watch, race starts are now much more closely scrutinized. For example, when the green flag is thrown at the start of a race, most series mandate that the cars must stay in two lines and drivers can't pull out of line to pass until they cross the start/finish line. Since the green flag, which is only thrown from the starter stand, is dropped somewhat before the actual start/finish line, there are some drivers who are going to get a better start than others. As such, they will be closing in on the cars in front of them before they get to the line. "Too bad how sad," say the rules. You can't make a move to go around that driver in front until you go by start/finish. It can be immensely frustrating if you catch the driver in front sleeping, but it can also save your behind if you balk the start. Once by start/finish cars can fan out, but by that point, pretty much everyone is going the same speed.
There should always be that thought of risk versus reward once the green flag drops. Just because you really, really want to get by somebody doesn't mean that it is possible, and many times trying to force the issue can give other drivers behind you the advantage. In no time, you can go from a well-earned qualifying position to several places down the ladder or even take yourself and others out of the race with an ill-advised maneuver. Like so many chief stewards have said multiple times, "you can't win the race in the first corner."
Having said all that, it is true, especially as you get to the higher levels of the sport, that winning the race into turn 1 becomes more and more important. And there is the fact that some drivers are better than others at "reading" the flag marshal and sensing the timing of the green flag, or just plain get lucky. I remember qualifying on the second row in the Koni Challenge at Mid Ohio in 2007 in a 40-car field and feeling pretty darn good about myself. It was short-lived, though, because as we headed into turn 3 after the start (the start of the race at Mid Ohio is always performed on the back straight just before turn 3), one of the Compass360 Honda RSXs came blowing by all of us. Whoever the hell it was, they had timed the flag to total perfection, and went from seventh to the front of the field before the first turn. Two things to remember on this: 1) such results while spectacular are certainly not typical, and 2) racing is the kingdom of hero to zero, as I spent the rest of the race wondering how in the wide world of sports he had caught me and the rest of us snoozing like that.
Another reason that a green flag can be thrown is for the restart of a race that has already begun and has been "paused," so to speak. If something happens on track where the circuit is partially blocked or where emergency vehicles are dispatched (car upside-down, driver walking back to pit lane with just the steering wheel in their hand), the race is slowed and the cars are regrouped. Commonly referred to as a full course caution, which we will discuss in depth at another time, the subsequent slowing of the cars allows a safer environment for emergency crews to work on the vehicle(s) and driver(s).