Dungeons and Dragons is based on many works of fantasy fiction. Whether its Lankhmar, Conan, Dying Earth or Middle-earth, they all have a hand in its creation. But Dungeons and Dragons isn't set in any of those worlds - or at least wasn't for many years. There are versions of D&D set in some of those worlds - but D&D as an entity stands on its own. Not long after D&D we saw The Call of the Cthulhu from Chaosium, which to my knowledge is the first role-playing game set in a pre-existing world established by an intellectual property.
Since the release of CoC, it seems like any IP that could be turned into an RPG was. Star Trek, Star Wars, Marvel Super Heroes, DC Adventures, Dune, Dragon Age - the list goes on and on. Games set in pre-existing worlds with an established canon familiar to the table top gamer have the advantage of providing an immediate and shared understanding of the setting. This makes theme and tone easy for a game master to establish and game mechanics can be tailored to fit aspects unique to that setting.
But, there's a trade-off: Canon. In Star Wars Luke Skywalker destroyed the Death Star. In DC Comics, Superman is pretty much a god. Players want to play in these settings to emulate both the setting itself and its heroes - but often the player characters can never be as good as iconic character, or at least are limited to accomplishing lesser tasks than the famous heroes.
Does this make IP settings self-defeating? I sit down and play Star Wars because I want to be like Luke Skywalker - but I'm never going to accomplish the things he does, so is my goal truly achievable at all?
The answer is "Yes, but..."
In theory your character could participate in the Battle of Yavin and be the one who destroyed the Death Star. But in doing so and undermining both the established canon and the established character does that cause a deviation from the IP so great that the IP is no longer truly what it began as? If someone other than Luke blows up the Death Star is Star Wars still Star Wars? After all, now the familiarity that provides comfort and understanding for a given setting has been shattered. Where do they go from here?
So, in IP settings is it worth breaking canon and potentially irrevocably change the setting or can players be content to live in the shadow of these glorified NPCs? Is there a balance to be found?