I think it's threefold. I'm sure there are more advantages, perhaps you'll point some out in the comments. But for me, these rise to the top of the list:
- Escape. Let's face it, life can be boring sometimes. I've always preferred to fly around as a superhero, or disappear into an enchanted forest full of unicorns (in my head of course) than do let's say... homework. Not that I didn't do my fair share. But if I had the choice with no consequences? No contest. Who doesn't want to be the hero?
- Safety of Symbolism. I don't like preachy books. Sorry. I don't want to be told what to do or think, no matter what the source. I prefer intelligent discussion and thought provoking ideas. So when a book showcases a very real issue or situation and explores some of the finer implications in an honest way, I'm hooked. And fantasy let's us explore these themes in relative safety by changing the setting and characters to something unfamiliar and therefore less threatening or obvious.
- Playground for the Imagination. Yes any story builds a new world, but where else can the sky be purple and the butterflies poisonous? Hey that's kind of cool... *runs to write it down* The possibilities are truly endless.
Everyone's taste is different, which is why it's so wonderful to have choices. Sometimes it's too hard to suspend disbelief. But I believe there is some fantasy out there for almost everyone. Many have read Harry Potter who claim to not like fantasy at all. What do you think? Do you see any additional advantages of fantasy as a medium? What about disadvantages? Share!