From the Google AdSense Blog:
What can I do with the AdSense API?
Using the AdSense API, you can enable users to perform a variety of AdSense functions without leaving your website, including the following:
– Create an AdSense account
– Manage an AdSense account
– Create and modify AdSense for content ad units and link units, AdSense for search boxes, and Referrals
– View detailed reports to monitor performance and earnings
How does the AdSense API benefit your site and users?
By making it easy for publishers to sign up for AdSense and generate revenue, the API offers another compelling reason for publishers to choose your service over a competitor’s–and remain loyal to you. The AdSense API is great for publishers who don’t want the hassle of setting up their own accounts or dealing with cutting and pasting HTML snippets.