One could imagine a scenario where you’re looking at the duration of some brief event by looking at the start and end times. If that’s interval happens to span

One could imagine a scenario where you’re looking at the duration of some brief event by looking at the start and end times. If that’s interval happens to span a leap second then the duration could be significantly different depending on how your timestamps handled it. Much more important, though, is how it affects the future. The fact that timestamps in the past might be a few seconds different from the straightforward “now minus N seconds” calculation is mostly a curiosity. The fact that clocks might all have to shift by one more second at time point in the future is more significant. There are plenty of real-world scenarios where that needs some substantial effort to account for.