From Wikipedia:

A click tag is a parameter used in Flash banner ads. The parameter is a variable that defines the destination URL from the markup code. By using a click tag, the advertiser can easily see and modify the URL without a Flash developer.

The click tag enables the ad serving network to gain metrics such as the amount of clicks and views or which sites these clicks and views have been made. By reading the data gained by the click tag parameter, an advertiser can evaluate how effective the ad campaign has been.