User, yes you are right. Many Preps however don't exist purely to get kids through the Kent test but also have to focus on getting kids into e independents too - so the focus isn't as clear as it could be - children may do less prep for the exam type they need and spend some time on other types that are not useful for them.
Also, preparation in a group or class at Prep never gives you the 1-2-1 focus of a tutor. The explanations and picking up if weaknesses by a teacher to a class may suit an individual and their needs or not. A tutor is totally targeted. This is why people with kids in great Preps also go for Tutors - because they want that 1-2-1 targeted approach that you cannot get unless you have a tutor or you do theorem yourself. The best Prep in the world is still teaching classes and not individuals.
Good state schools are perfectly able to cover the maths and English on the NC to a good standard. A tutor can cover any yr6 stuff needed which hasn't yet been covered and plug any gaps. Bearing in mind the £1000s saved by using tutors instead of Prep schools (esp if you need tutor too) you cnq see why loads go for this option.
I suppose some people with very average children might decide their kids need more than a state school will provide. For kids who really are very borderline and not naturally in the top 30% but perhaps at 50% parents might decide that a prep will give them the boost to get them to standard - well they might or they might not. Preps and tutors can do some polishing, but excuse the expression, you can't polish a turd - some children simply will never pass the Kent test and that won't be the failing if the school or tutor - it has to be accepted as fact.
Personally my conclusion on Preps is that a good Prep is worth paying for, for the broader stuff which happens there - the 3 afternoons a week of sport, the great language teaching using specialists, the off-piste curriculum freed from the NC and the solid academic teaching. I think it's worth paying for that. I honestly don't think they do loads more for a kids maths and English than a good state school can do, if a tutor is also used. In fact, the 1-2-1 focused approach of a good tutor which gives very targeted help I think can achieve more in an hour in terms of uplift, than probably 3 hours in school on English and 3 hours on maths in a lovely small class of 16, which is still a group situation and not targeted 1-2-1 help. Pick the Prep for the wider stuff - yes, but don't think they can deliver loads more in just English, Maths, VR and NVR than a good state school plus tutor can. Just my view from seeing kids at state schools with tutors and prep school kids with tutors and without.