Spoiler alert: most things people buy they don't actually need.
As you correctly observe, the statement "people wouldn't buy what they need without advertising" is obviously wrong. What I was getting at was manufactured demand, and my belief that it's a significant part of non-immediate-need purchases.
I don't have any disagreement with the claim that some demand is advertising-generated (in fact, it's a non-trivial part of advertising's purpose). Thanks for clarifying!
https://www.britannica.com/topic/conspicuous-consumption#ref...