Google Webmaster Tools will tell you of some broken links, but not all; it's not proactive about informing you, whereas Relink will be scanning regularly for this specific purpose and telling you. I'm also interested in making sure that all links that ever worked continue to work, something GWT doesn't do. Those are a few of the things; I've got much more in line for it where GWT is not competition at all. You may find more useful info in my post from yesterday, https://news.ycombinator.com/item?id=6489084 (the actual submission of which became [dead] for some reason entirely unknown and unclear to me).
