Young kids especially, are still developing emotionally. For a basic example, so whilst they may be able to recite their times tables to the nth degree, they might not be able to handle a situation where there's a group of kids, and someone steals their toy. That frustration can come out in a lot of ways, maybe the kid starts to shut down and stops talking, maybe they regress in their potty training and start soiling their pants again (quite a common one, it's something they can control and no one else can) etc etc. At that age kids are really only coming to a point where they understand the concept of self, and potentially haven't yet understood the fact that other people have their own concept of self. Pre-school + first years in school, are basically the age where there are the most aggressive/violent encounters for pretty much any age as a human. Older kids (think 15/16) may express this violence more dramatically, but you're still often measuring it in terms of episodes a week, maybe a day, for little kids, it's episodes per hour.
They simply don't yet have a complete handle on who they are, let alone who the rest of the world is, and how to handle that. We can give them skills to help with it, much as we can give adults skills for the same reasons, but sometimes it's just a matter of simply being too young.
Probably a more common experience is that of young teens, whilst some in the group are powering into puberty, others simply aren't, and that also means a large difference in emotional control and maturity.
My missus did a lot of childcare stuff before, and where we were, if a child wasn't felt to be ready for school emotionally, they were simply held in kindergarten for an extra year. No big deal.
As someone who skipped a year at around the age of 9, and was late to puberty anyway, I can fully understand the importance. Perhaps not so much at a later age, but at the very young ages (such as 3-5) when a childs emotional world is still developing so much, it's worth considering.
I would add, it's not something everyone has to panic about, just because their child is on the younger scale of the school year, but it's worth a moments consideration. The vast majority are fine.
I'm not qualified, educated etc in all this, and may be getting bits wrong, but the general gist at least is what I'm trying to suggest.