The key to DVC is the flexibility that comes with it, as others have said. It is basically a Disney timeshare program. Your initial purchase becomes your "home resort" - but you will have the opportunity to use any of the resorts in the growing DVC collection of resorts. There are 6 DVC resorts at WDW (soon to be 8), one at Vero Beach, one at Hilton Head, one under construction at
Disneyland and one in initial site-work (pre-construction) in Hawaii.
At your home resort you will be able to book reservations 11 months prior to your desired vacation time - all other resorts it's 7 months prior, so the priority for that individual resort belongs to those who purchased that resort as their home resort.
You also purchase a # of points per year vs. a set week each year. How you use those points is completely up to you. You can use them all at once, or over several trips if you want. If you won't be traveling one year, bank them into the next year and enjoy a longer vacation, or several smaller ones. Or, if you need to, borrow points from the coming year to give you what you need in the current year. Plus you can choose the size of your accomodations based on the needs of an individual trip - perhaps one time you need a 1 br and other time you only need a studio, maybe another time you want to bring some friends along so you need a 2 br - ok, as long as there is availability you can do it!
You also have the flexibility, if you want, to trade out those points to use at other Disney resorts around the world (including FL, CA, Tokyo, Paris and Hong Kong), or trade-out for one of hundreds of resorts via the RCI timeshare network. You could also use the points for Disney Cruises or Adventures by Disney excursions, if you so desired. Any of these options would come with a $95 transaction fee.
As an owner of points, you are responsible for annual dues in proportion to your point total, but when you stay on points at a DVC resort, you don't pay anything out of pocket since you've paid your lodging expenses in advance via your initial purchase price. Basically you're pre-paying for lodging via your purchase, and then get to enjoy the benefit when you want to and how you want to over the life of the contract.
There are perks and discounts that come along with your membership as well, but they can and do change from time to time, so don't consider joining just because there might currently be a cool perk. Currently there are discounts on AP's at both WDW and Disneyland, and various other discounts.
For us, having the room size was important. As a family of 5, we didn't enjoy cramming into a small hotel room. Being able to spread out, make meals in our room and having the flexibility of going when and where we wanted to sold us on DVC.