When placing a link in a mass email, or placing a link on a third party location, you can set a parameter in your URL as it will allow you to do more in-depth reporting in your analytics and Calltracks.
For example, when you send an email out to a list of clients or customers, you will want to track how many of your clients click to your website, and then call up. Inserting either a wnd_source or utm_source paramter will allow you to see that the caller originated from the particular email campaign enabling you to understand how effective the email was.
The existence of this tag will ensure Calltracks sets the WND Source parameter for the call to this value. This may differ from, say, Google's opinion on the Source but at least you know that Calltracks attributed the call to this source because you placed it in the URL.