The on property resorts provide several major advantages. The first is somewhat ephemeral. All the WDW resorts are intricately themed, immersing you in the Disney atmosphere 24-7, creating a total experience that cannot be equaled elsewhere.
The other reasons are more concrete. The location of the resorts allows for easier access to the parks. You will save travel time to and from the parks everyday of your vacation. You will have the option of returning to your room for a mid-day break more easily than with off-property resorts.
Depending on the off-property resort you choose you may have to pay for parking everyday if the resort does not offer a shuttle service or the service does not fit your needs.
The Magical Express and WDW transport completely eliminates the need for a rental car.
The option to purchase various Disney Dining Plans will save you money if you plan on eating in the parks and resorts for the bulk of your meals. These options also allow you the convenience of knowing what your food cost will be upfront and not have to worry about overspending.
There are many smaller perks such as the ability to have purchases made at the parks and in the resorts sent back to your room. The key to the world that includes your room key, park tickets, and dining package all on one card. You can even attach a credit card to it to charge other items. You will also have the opportunity to have more time at the parks through Extra Magic Hours.
There is also the difference in service. Disney cast members are trained to provide over the top service, most of the time this is the case, and when it isn't I have found management is extremely responsive to legitimate complaints made at the time.
