Not trying to start a flame warm etc, assuming some of you can take it for what its worth and not start a fanboy contest in either direction.
Just going to leave this here: Channel9 MSDN
While texture tiling has been done in software before, it had certain limitations. By moving it to hardware the limitations were removed. Without going into all the technical details, in short, the benefits of removing these limitations are so impressive that it allows developers to store texture data sizes that previously took up 3GB of RAM in only 16Mb of RAM! Not only does it offer a drastic reduction in size, but it can also allow more detailed worlds than before since now developers have a lot more texture storage available.
In addition to textures, and presumably the reason Microsoft prefers to call this partial resident resources, is because this technique can also be applied to other areas such as shadows through shadow mapping. It's demonstrated in the video at 22:40.
Pictures and other links, as well as the full explanation can be found at the link above.