One way to look at it is that “speed”, in km/hr, is an instantaneous measure. As you walk, you speed up and slow down; maybe there are hills or something. That’s not super useful to most people. What is useful is knowing “is this chunk of my walk/run slower or faster than the last chunk?”. Someone running a 10k wants to know their average speed for each kilometer. Someone running or walking a shorter race might want finer splits, but it’s convenient for comparison to still use 1 kilometer as the baseline because you have a better sense of what it feels like to walk a 12 minute km pace than a 6 minute half km pace, even if they are the same thing.