It can serve multiple purposes, though legally is it's most predominant. If by chance anything referenced has bearing on a real life thing, one can perceive the work as factual or at least an unflattering commentary/slander against that thing, resulting in legal prosecution. In this instance it serves the same purpose of putting "Caution Beverage Hot" on a coffee cup.

Another use case I can see is in cases of historical fiction, in which while the events are indeed based around historical facts, there is allot of author liberty taken in telling the story, and to dispel the idea it is indeed 100% fact, a clarification is given.