It is not likely useful to them in negotiating ad rates, at least not with how advertising is usually bought/sold, because the broadcaster has almost no information (if any) about anyone watching these streams, including if it's even a person. It's possible they could fold it into a more general ad package that places a lot of weight on total view/viewers numbers, but that is a lot less common these days, and when it is done, the rates tend to be much lower