For example, if you tell a typically developing child to brush their teeth or they’ll be no TV I guess when they’re older they’ll still understand that brushing your teeth is important and won’t need someone there imposing a threat in order for them to do it. I think for some children if you don’t explain why it’s important (often many, many times) when they’re older they’ll still need someone there with a ‘threat’ in order to get it done and they won’t achieve independence.
I think this is very simplistic, although appreciate that there is a limit to the compexity of an idea which can be expressed in one sentence in a forum post.
Teeth brushing is a great example though, because it is something which doesn't really have an intrinsic, instant feedback built in. It's difficult to tell if your own breath smells, and it takes quite a long time for toothache to develop after not brushing teeth, and the damage is already quite severe by the time the teeth actually hurt. Plus, we only get two sets of teeth and the second set could be damaged if the first set are extremely poorly cared for. So parents often need to use a variety of ways to persuade children to do it, unlike for example walking, which has a clear and obvious benefit and most babies/toddlers will be motivated to do it on their own.
Most things parents do in terms of tooth brushing are the set up work which happens long before children can be motivated by either extrinsic reward/punishment or reasoning, when they are babies and toddlers - we make it part of the everyday routine, we may do it in some format even if they are protesting, we try to make it fun/positive using e.g. songs, games, a toothbrush with a favourite character, a toothpaste with a flavour they like etc, we ensure that they see adults modelling the behaviour, we express approval and admiration in response to tooth-cleaning.
The age range you're talking about, where we can appeal to reason/logic in order to build intrinsic motivation, or use external reward/punishment (or approval/disapproval) to create extrinsic motivation, is the stage following this. Children in this stage haven't yet developed the ability that adults have, to understand a far-away consequence and use this to modify their own behaviour or guide their decision making. However, everyone develops this ability at different times and some people find it easier than others. It's related to executive functioning, some of which is developmental, so it is universally true that the younger a child is, the more immediate the "effect" needs to be for them to link the "cause" to it. Older children can link slightly more distant cause to effect, and adults can do this best of all. But even within groups of children the same age, some of them can modify their behaviour in response to something they anticipate later and some will find this more difficult - you might have heard of the marshmallow test? Essentially it tests a proxy of the same ability. The ability to defer gratification in pursuit of a later reward.
So because it is almost developmentally impossible for children to grasp a consequence as far away and serious as tooth decay, let alone for them to use such an abstract-seeming thing to self-motivate until they are much older, parents have to do quite a lot of what is essentially "translating" this far away, serious consequence into something much closer and more tangible, and that is the role that unrelated consequences can sometimes play.
They won't need this forever, because they will reach an age where they can understand the long term consequences (and care more about the short term social consequences of having bad breath) and make the choice for themselves based on that reasoning. Or it will already be a habit by then which (hopefully) makes it easier for them to continue the behaviour into adulthood.