I always thought it was a package thing (hotel, tickets, dining plan), but someone here has gotten it a la carte with a room-only reservation.
However, like others have said, it's not really worth it. There are a few people on here who do like it, but it's ONLY a way of pre-paying and (IMO) perceived less hassle. Unless you plan out your dining exactly, you're basically just paying dollar for dollar, with the possibility/probability of paying more or losing some of the money you paid.
I personally like figuring out my dining budget and then putting that on a giftcard; either one big GC or divided it up and get a smaller GC (or just refill one) each morning. That way it's prepaid and very simple.