Money - The Driving Force  or the root of evil, whichever way you prefer to view the statement. But without a doubt, it is the primary reason that dictates the existence of many popular websites today. Pick any community oriented websites from your bookmark, and good chance the site will be spammed with advertisements of all sorts (including this).

How profit is profitable you asked. We use example of pay-per-click where advertiser pays you for every click on the advertisement by your visitor. Lets assume you are writing a book review where advertiser pays you $US0.05 per visitor click. Your site receives an average 1000 unique visitors a day. And suppose 10% of your visitors are interested to purchase the book and click through the ad, that amounts to a decent:
     (30 days x 1000 x 10% x $0.05) = US$150

Multiply that by the number of reviews you will be writing, and increasing popularity of your site (engadget.com gets 3 million hits a day). You do the math.

Business Model...