Why do people run with a buggy?

Genuine question, as someone who doesn't have kids I can't really answer it. What benefit does it have for the child? Is it just that nobody else can look after the child so this is a way to still be able to parkrun? Is it a personal challenge for you to make the run harder? I understand the logic to running with a dog as it is exercising the dog, but I've never fully grasped the buggy thing. Just for clarity I am not saying it's stupid or saying it shouldn't be done, I just wondered why it has become a thing and what reasons people have for doing it.