I'm not really sure how they can help but I guess one of the reasons below could be why;
1). They are more shorter and so can be used easily on forum signatures. Since the signature feature is always limited to a short amount of texts. Also short links can easily be promoted on twitter compared to longer one.
2). By using a short link, the people you are promoting the link for would have no choice but to click on it to check it out. However, if you use the actual link, they can memorize the url and leave out the part that link you as their referral. So, they can visit the site later.