Yes, nothing happens until you trade a dollar for something, but it does not have to be this site they spend money at.

Advertising isn't even about getting people to open their wallet, it's more about influencing their decision when they do go to spend money or make a purchase.

No? Advertising money is paid upfront. X number of impressions. You get paid a cut for hosting the ad. The ad might be a huge failure and lead to zero clickthrough or purchases. But the money has already been paid for the campaign.