The point of Google (or, of search engines, at least) is to surface content relevant to a query. If the content exists, it should be surfaced, and it should not

The point of Google (or, of search engines, at least) is to surface content relevant to a query. If the content exists, it should be surfaced, and it should not require an advertiser to suggest you surface it to the user. If the content is not surfaced except by ads, that's a failure case for Google (or, for search engines, at least). Saying "Ads improve the user experience of a search engine" could only be true in the case where the search engine is already failing, and one might suggest spending the money and resources one is spending on an ads engine on improving the search engine, if the search engine is actually trying to optimize for delivery of relevant information. There's certainly one metric that'll always be improved by showing ads, though, and I suspect it's that one, and not any of the others, that's driving the decision to put ads on the page.