It's insane. If companies are buying ad-space, it's because they expect to get more business in return. This means that someone out there is being influenced by said ads, so that if the content cost X to put up online (hosting, funding its creation), someone is paying X+(ad company overhead) for it.
If these costs are being borne evenly, then it's complete societal waste. We could pay X for the content, and not incur the overhead. If these costs are not borne evenly, and some people are paying for the consumption of more disciplined people, it's probably contributing to terrible cycles of poverty (ie: some kid spending money on fancy new shoes he doesn't need and can't afford is paying for a well-paid tech-users YouTube habits, because it preys on their lack of education). Either way it's terrible.
Advertising isn't free. Insofar it works, for some people, it's basically coercive via psychology and simulated peer pressure.