If I have a blog that is getting 100,000 page views a month that means that I’m probably getting at least 50,000 people to the site (most blogs will do between 1.2 to 1.4 pages per session). That means I have to try and get some small percentage of those people to buy something from me if I really want to do well. If I can’t get them to buy something then (in some cases) I have ads running on the site that will make me money anyways.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.