But this is not always a viable performance metric when the agency cannot fully control all aspects of the customer acquisition and conversion process.
Advertising with the purpose of generating a lead (which may be defined in a variety of ways), may not always result in a customer because there might be a number of steps after response to an ad and between first contact and final sale.
P4P advertising became popular with the advent of the internet that allows real time measurement of a marketing campaign's ROI (return on investment).
As mentioned above, a caveat to P4P advertising is the degree to which an agency can be held responsible for outcomes while controlling the variables involved.
These are not direct ad creation issues, but align more with brand trust and expertise, landing page conversion effectiveness, and more.