In some forms of GT racing in the US, multiple classes run together in the same race. The yellow headlights allow drivers to see whether a car behind them is in the same class as they are. Because the yellow headlights are usually in the slower class, it's usually a matter of a driver in the slower class seeing WHITE headlights in his mirrors and being able to tell the car behind him is in a faster class. In short, it's a way for drivers to tell whether or not the car in their mirrors is one they're actually racing for position with.