It's horses for courses. I DIY'd with my son who has places at QEB and Bucks. Tutors have their place, and for DP's who do not have the time, or are not confident in their abilities, a tutor is a good option.
I think it would be unfair to send a child off to do an exam which will dictate how that child will be educated for the next 5-7 years without first preparing them for it.
The reality is that only a small percentage of children will end up in a grammar school, and that percentage is unlikely to change. Tutors will not affect that percentage. There are only a small number of GS places available, and the competition for them is very high. QE only has 180 places, yet over 2,000 boys sat the exam. That means that only 10% can gain a place. This has nothing to do with tutoring, it's simple availability. If fewer sat, or there were more places, then more than 10% would gain a place.
Parents are simply trying to do what they believe gives their children the best possible chance of gaining a place at the schools they think are best for their children.
VR and NVR are not taught in most primary schools, but are usually examined at the 11+. I can imagine that lots of parents believe that they are best paying for someone with experience to teach their children, rather than trying to do it themselves.
It is clear from other posts that not all bright children do well in the exams, it is also clear that children who are not quite as bright, but have been well prepared can do well. It is certainly true that not all tutored children gain places, but then neither do all children that were not tutored.
Some children respond better when they are being taught in the more structured tutoring environment. Others, like my DS would prefer to be taught by their parents. I don't think it is fair to make general sweeping statements Biut the value or otherwise of tutoring. For some it will be perfect, for others it will not. It depends very much on the child, parents and tutor.
As stated earlier, my son has gained places at GS, and we did not use a tutor. However, I would not criticise those that did.