It's classical motosport graphing. Just showing split times helps to break down incremental performance for each competitor, but lacks the context to know if someone is shortening a gap over a succession of splits. The arrow (typically red or green depending on whether falling back, or advancing) shows their trend over timel, often updated per lap, but sometimes (as in GT4 it seems) updated continuously. So, if someone is closing the gap, it's just a matter of time before they catch and pass. Knowing ahead of time that they're creaping up can alter your strategy, urging you B-Spec drone to press a bit harder. Say for example you hammer along at spd-5/pass to get in front of the pack then kick back at spd2 for a while, you might notice that the #2 car starts to gain (shown by shorter splits over time, as well as that big arrow) so you'd want to step it up a bit to keep a comfortable lead. At the same time, if you're trying despirately to catch a leader and still get that arrow pointing backwards for each split, you know you're done.