Tuning and engine diagosis. If you had a leaky injector you might not get the temps as high as they should be from runing rich, when you're dialing the car in you look for the hottest temps you can get and then fatten it up a little for safety.

EGT is far better than a/f for getting a motor in its groove. Once you get it there, you'll look for consistency of the temps in similar conditions.