The upshot is the lower the DA, the denser the air is, with more O2 content. Cold, dense air will make more HP than hot humid air. So on a night with really low DA, the car will run stronger and will post a lower ET.
Calculating DA at the same track is important when evaluating other changes to the car, like intake, tune, exhaust, etc. It's also important for bracket racers when predicting their ET at the temps drop and humidity changes over the course of the day/evening.
It's also a way to normalize ETs for the same car driven at various altitudes. A 392 run at a very high altitude track where the air is really thin will post considerably worse times than a 392 run at a low altitude track. There are formulas available to correct the ET for the DA at the track, referenced against a standard DA. Sometimes (usually) the corrected ET will go down, but sometimes it goes up too (like if the DA was negative at the track that day).
