Because it allows me to plug a Droid into a computer running iTunes, and have it automatically synchronize the 2 devices with the iTunes playlist of choice. If I delete something on either side, the other side is updated to reflect the change. It makes file management a lot easier than dragging and dropping, particularly if you have a lot of songs/albums that you're trying to manage. In my case, I have a specific playlist in iTunes called Droid X. I usually select multiple albums from my music collection and drag them into this playlist. From a file management standpoint, I find this a lot more user friendly.
Another advantage, is that it pushes the album art over as well. I haven't always had the best of luck with album art using the drag and drop method.