One of the most common goals in RPGs is to "save the world." Often this isn't literal, you are merely tring to save society as it is then known. When one takes the ide amore literally examples of Saving the World could incude stopping a meteor from hitting the world.
In the RPG Final Fantasy VII by Squaresoft, the player literally saves the world from utter destruction, which does not happen often in "Save the World" scenarios.
Some players like the epic feel that Saving the World gives an RPG.
Others feel more motivated to complete a game where the storyline involves a task of global importance than they would if the task was something more mundane.
Some people feel that the idea of Saving the World has been used too much and has become cliche.
Others feel that Save the World plots are unrealistic.