Of course these days the line is often blurred. So half the posts on HN could be adverts, and you wouldn't really know about it. In fact lots of them are adverts.
But just think about how the world would work without advertising. How would you know there's a star wars movie coming out? How would you know about new products and services you might be interested in buying.
I buy quite a few magazines, and one of the reasons I buy them is for the adverts, which tell me about companies who provide things I might be interested in.
For example, I buy a bee-keeping magazine, which has many adverts related to bee-keeping. That's valuable. I buy a pig magazine, which has adverts for pig arks, pig tags, weaners, etc etc.
Good advertising is a net win for everyone. It provides us information about things we might like. Just because there's some bad advertising on the internet, it doesn't mean all advertising is useless.
What is the alternative model to advertising on the internet? The fact is, that most websites are supported by advertising, and if that goes away, so do the websites unless some other magical income model replaces it.