why also would i use it and for what?

TrackBack is a type of peer-to-peer communication system that was designed to send notification of updates between two Web sites via a Trackback Ping. Ping in reference to TrackBack refers to a small message sent from one Web server to another. TrackBacks are useful for informing a Web site that you have referenced its Web site within your own Web site, and is popular with bloggers.
TrackBack was first released as an open specification in August 2002.

You would only use it if you designed a website and you used code that someone else wrote. Any times you will go to a site and download something only to go to another site and see the same download. Being a web designer myself I have to shut down the links or sites from using my code without my okay. Trackback allows those sites to just communcate with me. The designer might have only used my code to better improve his site or mine. So its a needed tool in a web admin world. Below you will see the true diff of what a trackback url is.

Trackback
Trackback enables you to notify another site that you wrote something related to what is written on a specific page, even if you don't have an explicit link to this page. This improves the chances of contributors to this page noticing that you gave them credit for something, or that you improved upon something they wrote, or something similar. With pingback and trackback, websites are interconnected. Think of them as the equivalents of acknowledgements and references at the end of an academic paper, or a chapter in a textbook