I don't get it - conceptually isn't a site/service provider selling the ad space to the highest bidder already? Isn't the goal to max out that revenue?

Maybe allowing your "best" consumers to opt out of ads makes overall CTR drop to some level below which advertisers will no longer want to use your site.

Any insight into numbers for this? It is way outside my knowledge.

I'm guessing, for example, if I said to you - hey, I like your site and will pay $1,000,000 a year to see it without ads that might be a deal you would take?