Trackback helps you to notify another author that you wrote something related to what he had written on his blog, even if you don't have an explicit link to his article. This improves the chances of the other author sitting up and noticing that you gave him credit for something, or that you improved upon something he wrote, or something similar. With pingback and trackback, blogs 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.