Board Thread:General Discussion/@comment-26479628-20140506173222/@comment-1038387-20140506175715

Basically, it allows you to categorize it under something else. If a page is called "The Whatever", you can DEFAULTSORT it to "Whatever, The" (or just "Whatever") so it gets categorized under W instead of T. The same goes for full names: "Hawk, Joseph" makes it appear under H rather than J.