When you use a service that is funded by advertising, the service only gets that funding because the advertisers trust that they are getting their ads in front of real users. Some advertisers are able to precisely measure the quality of their traffic, for example by seeing whether the traffic they get buys things, but most are in businesses where that's not possible (no one clicks on an ad for Coca-Cola and then places an order for Coke). Ad fraud means that advertisers are less willing to pay to be shown on the service, so the service's funding decreases.
Very likely, less funding for the service hurts you as a consumer: they are probably spending their funding in support of the site. For example, I believe that the ads here fund the moderators.
> why should you be allowed to spy on me - who never defrauded any advertisers - to fix your problem?
See my response to wil421: https://news.ycombinator.com/item?id=25430453 I think that fix here is some thing like https://blog.cloudflare.com/cloudflare-supports-privacy-pass... or https://web.dev/trust-tokens/ that allows detecting and preventing fraud in a privacy preserving manner.