Why would I want to Pay-Per-Click, when I can get free traffic on Google?

The main reason for wanting to pay for traffic is because it is very very difficult to receive free traffic on Google.

When you sign up for Google AdWords Pay-Per-Click advertising your advertisement will be displayed almost immediately in Google's search results (usually within 5 minutes). You can also decide which search results page you want your advertisements to appear on (the first page always receives more visitors than the other pages).

If you want to receive free advertising, you would have to own the website you were promoting. It would be really difficult to do otherwise. Then, you would have to optimize your website for Google and then submit it to Google to be displayed in their search. This may sound good but it can take up to 6 months and that will not even guarantee you a 1st page position.
Affiliate Marketing on Google AdWords

Contrary to what most people say about affiliate advertising on Google AdWords, it is not required that you have your own landing page or web page in order to advertise for another website on AdWords. Most people think they need to have their own landing page because Google doesn't allow two or more advertisements for the same web page to appear for the same search. This is not true. In fact, videos included in this guide prove this isn't true.

Here is how this rule works: when you create an advertisement for Google you are asked to enter two different types of URLs (web addresses). One is the Display URL which is the URL that the web surfer sees on your text advertisement and the other is the Destination URL which is the actual URL that the web surfer goes to when they click on your advertisement.

Google then looks at these URLs and compares them with your competitors URLs for each keyword in your keyword list. If there is another advertisement that uses the same display URL, Google will not display your advertisement because there is already another advertiser advertising the same website as you for that keyword. Google will only display your advertisement in place of the competitor's ad only if your text ad is better than theirs. Google uses an algorithm called the Quality Score (we talk about this later) to determine this. So if your quality score is better than the other advertisers, then your advertisement will always be shown.

There are two ways to always have your advertisement shown:

1. Create your own landing page. If you do this your text ad's display URL will be unique, so your advertisement will always be shown. Unfortunately, using this technique requires that you know how to make a webpage.
2. Optimize your advertisement so its quality score will be higher than your competitors. This is the best approach in my opinion.