Publishers and advertisers use adnetworks for various purposes. In a simple case Publishers use adserver to store Text as well as Image Ads. These Ads are rotated by for increase optimization and performance for overall spending.
The Ad servers are configured so that they are able to record impressions, clicks and other details of the Image banners. Also the Web page code need not be changed every time the Ad is changed
Additionally most important factor is the option of retargeting the image ads for a given user. For instance, the users can view different ads when he returns to same site based on his Location and user demographic profiling.