What do you expect a Blu-ray player to do normally? Well, play Blu-ray discs, I imagine. Hitachi wants them doing more and decided to integrate a 32GB solid-state drive into the package. Yep, flash memory is so small now that we can turn optical drives into storage drives.
Hitachi calls it the "Hy Drive," and the idea is that the operating system and critical software components could be installed into the piggybacking SSD, leaving your usual platter hard drive free for your data. It's the kind of install setup the computer-savvy would get going with partitions on a single hard drive, and it could improve your overall system performance.
In reality, it's probably not something anyone will notice when Hitachi launches the new drives in August, but hey — if there's really that much of a performance boost, it could be a harbinger for all Blu-ray drives that follow it.