Apart from some token flights on Ryanair Prestwick doesn't figure anymore as Glasgow/Edinborough have their own airports far closer and more convenient. But back during WW2 when built there was a good deal of traffic. I suppose around D-Day invasion when every drop of airplane fuel was precious, C-54s from US came in from Iceland with enough fuel to fly back there.
Yeah the commercial stuff going through Prestwick is not what it used to be although it is used occasionally by other airlines as base training/circuit practice what is always good to see.
Prestwick is still far more convenient for the majority of us that live in Ayrshire, saves the drive to Glasgow or Edinburgh and through security in no time! can't get that anywhere else