Straight HD video on a Blu-ray Disc is called BDAV. It's similar to using your set top DVD recorder or VHS deck to record a TV show.
DVDit Pro HD uses HDMV in addition to the standard MPEG-2 HD video format. HDMV allows for authoring HD video and streamlines the process, as HD production incorporates many identical phases. It offers improved navigational and menu features, improved graphics and animation, improved subtitling support and new features like browsable slideshows.
So while you could place straight HD video onto a Blu-ray disc as data DVDit Pro HD enables you to include interactive menus, slideshows, multiple audio and subtitle tracks, and all of the complex navigation you have become accustomed to in DVD.