The Advertising module connects pages on your website with an advertising server like Google DFP.  This functionality is required for sites that count on advertising revenue.  The advertising module integrates your design and theme with the way you package and sell ads.  The buzz today is ad targeting.  You need to target one ad to a reader on a desktop computer and another smaller ad to a reader on a phone.  Additional targeting may be needed based on the content area of the site or based on attributes of your reader.  This does not magically happen.  It requires integrating the website with the ad server so that pages request the correct ad.  It requires coordination with the configuration of your ad server to ensure that ads can be targeted to the required ad zones.  The Advertising module includes ad server integration code that looks at page attributes, browser attributes and reader attributes and requests the correct ad from the ad server.