Aside from the example of a driver ignoring a furled black flag, the reasons for throwing an open black are relatively straightforward. Blatantly driving another car off course, exceeding pit lane speed limits, excessive blocking and passing the pace car under a full course caution are all examples of infractions that will earn you an open black flag and the requirement that you come to pit lane.
Now, while I wouldn't advocate ignoring the black flag, there's a strategy that is often played out-this, of course, is for when you're higher up the ladder and there is more at stake. Many drivers will call in to their teams and have their crew chief talk directly with the course officials before making the decision to come in. Maybe they can appeal the black flag before serving the penalty. There is a certain risk to that action, though. And waiting too long before coming in can really backfire on you in more ways than one.
There is also the idea of risk versus reward. If you get the black flag five laps into a four-hour enduro where you are running 25th in a field of 30 cars, then it is a no-brainer to come into pit lane and serve the penalty. Seriously, at that point, what have you got to lose? On the other hand, if you are leading the Indy 500 with only 10 laps to go, and you get the black flag for a questionable pass on the pace car then it might make sense to stay out. If you come in with only 10 laps to go, you're going to end up a lap down and out of the hunt regardless, so you might as well stay in there. Even though I'm pretty sure that's never going to happen to me, it might happen to one of you readers out there, just like it did to Scott Goodyear in the '95 Indy 500. Did they give him, Honda and the newly returned Firestone the win that so many of us think he and the team richly deserved? Google it. It's a great story, and it had everything to do with a black flag.