A properly targeted ad is useful. By definition, if the ad works on you then it did something useful for both the advertiser and the person who saw the ad.

Ads obviously sometimes work, or else no one would do ads.

It does, however, help pay for the content you're consuming.

I already covered that in another post: https://news.ycombinator.com/item?id=5459646

ads also serve to inform you of the existence of products