It's usually more that the ad results in someone buying something they were already planning to buy from you instead of someone else. There's pretty much 3 type

It's usually more that the ad results in someone buying something they were already planning to buy from you instead of someone else. There's pretty much 3 types of online advertising, Intent based where you show an add when someone is actively searching for a place to buy the thing, remarketing where you show an add because you were able to detect they were shopping for it recently, and then the window shopping ones which just show you things you might be interested in (these are the lowest converting) Online ads objectively work though. Online ads basically drive the revenue of millions of small and medium sized businesses.