I was curious about this , so I looked it up.
The idea of indexing is to position the spark plug so that its gap is facing the center of the cylinder, angled slightly toward the exhaust valve. This is important because, as the piston approaches TDC, the air/fuel charge is being compressed. The charge or "mixture" is being forced toward the area of the spark plug—and normally, the exhaust valve. The speed of this force inside the combustion chamber may surpasses supersonic speeds.
When 1/4 mile times are measured in 1/1000 of a second, every bit helps and that includes marginal ones. This could make the differnce between winning and loosing.