Enable tracking of sponsor logo serves and clickthroughs
Surely one of the most basic aims of a web site is to promote sponsors and to demonstrate the value that their sponsorship has achieved.
So knowing the number of times their logo has been served and the number of click throughs would be useful (understatement).
Clicks can be tracked by a third party service such as http://bit.ly
You can then get impressions / traffic data from your control panel.