As I understand it, it' essentially a "header" that's written to the disk. This "header" identifies how the disk should be read by a reading unit. A DVD+R disk usually has a DVD+R header written to the disk, whereas a DVD-R disk always has a DVD-ROM header written to the disk. A tool that allows the booktype to be changed would allow a person to write the DVD-ROM header on a DVD+R disk. This will usually create a final product that will be compatible with more devices. This feature is already available for many other DVD burner models (i.e ricoh-based units, liteon, etc...).
Unfortunately, the PX-708A will only burn +R media at 8x, -R media can only be burnt at 4x. Again, this tool would allow users to be flexible in the media they use (i.e. using +R media at 8x speed) since they would have the option of making the media more compatible for the end user/device (i.e. writing the DVD-ROM header instead of DVD+R).
At least, that's my take on it. There's probably many other benefits to a tool like this than I can think of.