The main one is that you get to book DVC villas for a very small recurring cost (annual dues) . Other benefits that *may* come and go include:
-10 to 20% dining discount on a very small number of restaurants.
-"free" valet parking a several DVC locations
-$100 off annual passes
-ability to buy the Disney Dining Plan with a given DVC reservation (there is some debate on how much of a "benefit" this is)
-various discounts on tours, recreation, spas, golf, etc.