Pay-per-click, along with cost per impression and cost per order, are used to assess the cost effectiveness and profitability of internet marketing.
Pay-per-click (PPC) has an advantage over cost per impression in that it conveys information about how effective the advertising was.
Clicks are a way to measure attention and interest: if the main purpose of an ad is to generate a click, or more specifically drive traffic to a destination, then pay-per-click is the preferred metric.
Once a certain number of web impressions are achieved, the quality and placement of the advertisement will affect click through rates and the resulting pay-per-click.