"Also apply to any child who has not learnt to walk/talk/potty train etc by the average age for doing it.
Being average or slightly less than average doesn't mean something is wrong."
No, but if there is an issue then early intervention will make a massive difference.
SALT doesn't work as well later in life, there are children who have missed out on life changing intervention because assessments weren't done early enough.
The same is true of other therapies.
Yes most children will be assessed and they're just not hitting the curve on average, but don't need anything, but it's worth doing in case they're one of the few who do need intervention.