Running a website costs money. Some people also want to make profit, but that's beside the point. Micropayment solutions at present are sadly lacking - especially for international audiences. Don't forget also that you and I are a technical audience; I know at first my mother thought that websites got a split of her ISP bill when she viewed their pages. So advertising bridges a lot of these gaps.

Instead it is a serious question about why some advertising must exist and be tolerated.

I don't believe anyone is saying that you must tolerate advertising :-)