There are a huge variety of independent schools.
They range from the top performing academic hot houses which also offer great extra curricular, to those which offer a fantastic broader experience but less of the academics, to the very many which are mediocre or poor and offer only a limited broader experience and very average or even poor academics.
Sometimes there is a correlation between price and quality, but not always.
So some independent schools offer good value for money and indeed add huge value. But lots don't add any more value than a state school (and sometimes add less) so are extremely expensive and not VFM at all.
In the end, we all have to decide what we value and will pay for. We have to know what the free alternatives offer and compare to see if the extras offered by the independent are worth it. We won't all come to the same conclusions, even about the same schools.
Personally, I wouldn't pay for a school which offered only moderate or poor results. I could get better than that in a state school. For me and for many, great results is a fundamental requirement. But I do want more and I will pay for more - I want the broad range of extra curricular, loads of music opportunities and sporting and drama activities, plus DofE to Gold level for significant numbers, opportunities for careers and university advice, access to massive help during UCAS time, access to lots of academic support if ever needed, great pastoral care. I want access to a group of aspirational children who want to succeed and engage in opportunity, which will help my children develop the same aspirations. I want success to be valued and sought after and the success of other to be acknowledged and enjoyed as a community, but also a place where children learn how fortunate they are and to have empathy for others. I want a lot. And I am willing to pay for those things if they are available.
However, I absolutely wouldn't pay for a few shiny facilities which don't even hide poor results and a culture of a lack of aspiration.
Choices do very much depend on the alternatives. If the only state alternative is a total dive, then perhaps the mediocre independent suddenly seems more attractive. However, usually there is a choice of independents, so I don't really understand why people choose the mediocre ones...I think sometimes it's because they can't really afford fees and have to choose the cheaper versions. TBH, I think these maybe a fake speconomy or not worth the money. When people have great state options available, I think the independent has really got to be extra special to be worth paying. If the State offers great results and reasonable extra curricular, then the up,fit of broader experience and a fantastic culture need to be truly fantastic to make it worth it, unless money is no object. But I think people do value those non-academic things highly.
By the time you are looking at top 50 or top 100 schools, they all offer great results. Parents take great results as a given at this point. What people are looking for and paying for to be in the really top and special schools is the extras beyond the academic - something very special which creates an environment of enquire and enthusiasm and engagement, coupled with opportunities to allow these to flourish. It's hard to pin this stuff down but people will pay a lot for it and it's about a lot ore than grades.