This is different from Cost Per Click (CPC) advertising such as googles adwords / adsense programs. In those the advertiser pays a few cents each time someone clicks on their ad. Banner regardless if a sale is made or not.
The Idea of a blog , like any other newspaper or magazine, is to attract people to the advertising in order to make money.
Important to know is that unless your actually go directly to a company site, someone is getting commission on that sale by you clicking on the Advertising for that company. You can choose whom that person or company is by going to their site first and clicking on a link. These links in no way increase the cost to the consumer and do not collect any information except to create a link to tell the advertiser, such as Amazon whom to pay the commission to