Well, imagine you create a 1-person startup. You make the perfect tool, but you have no customers. How do you acquire them? You can try pumping your own personal network, organically market yourself every chance you get (like HackerNews comments or Facebook posts). However you can, with a relatively low investment, reach an order of magnitude more users with digital advertising. Your startup makes a great product, right? So you'll naturally acquire at least some of the users who see your ads.

People don't have perfect information, advertising is crucial to inform your customers that you exist and have something of potential use to them.

I have several times bought something from an ad because it clued me into a solution to my problem that I had not known of. If my cat isn't catching all the mice I keep having to get more cats and try to get them to stop fighting or suddenly I discover mousetraps from an ad and I get a whole new solution that I didn't know about.

You have a business or a product that no one knows about. So you need to make people know about it. Enter - advertising

Well, I was talking about "YouTuber", "Streamer" careers.