If you make the kind of content that costs a lot to create but almost nothing to copy, you have a dilemma: Set the price too high, and you aren't getting any money from the poor at all at all, even though they're willing to pay far above your copying costs. Set the price too low, and you aren't extracting as much profit from the rich as you could be.
Ads let you serve the same product to everybody, regardless of income, because they pay for the product indirectly, by buying the advertised products. Rich people have more money and buy more products, so they, in effect, pay more for the same content. poor people pay less.
I think this is a beautiful system in many ways. Ads give us access to all the world's music, plenty of educational content (on Youtube), including from the world's topmost universities, international texting, picture messaging, phone and video calls of unlimited duration, even some movies and TV. And it's all free, which it wouldn't be if not for ads.
If we banned ads, many people just wouldn't be able to afford to talk to their family or listen to their favorite artists every day.