The DO is an entity that exist in all realitites and all dimensions. In order for him to be free he must be free in all of those places at once. TAR is the only other place where that is possible, where all realities exist at once. As the DO gets stronger not only does the Blight which changes creation to the DO's image expand into the real world but the rest of creation starts merging as we see in the later books. The ghosts which the Hero's imply might live in TAR also begin to appear as this merging takes place. Also look at the use of NEED to find the Green Man and Ishmael's excessive use of TAR and whatever those created world's he enjoys so much are considered.