The files stored in the BD Data Utility is for Blu-ray movies that have Internet access. This saves the information from the disc to the HDD. If you watch the movie again, the “settings” you had previously are all still there. Different movies do different things, such as give you wallpaper, or have games, interact with the movie, or allow you to talk to others watching Blu-ray movies, etc.
It will tell you when you run out of space so you don’t have to worry.