This board is the prototype which [Deunan] has been working on in order to use an SD card in place of a GD-ROM drive. The idea is to fully implement the hardware protocol used by a GD-ROM drive so that it can be completely replaced. The end goal is to do away with the optical drive on a Dreamcast game console.
As these game systems age, the optical drive is the most likely part to fail first as it involves moving parts and a lens that may degrade over time (we’re basing that assumption on our experience with DVD-ROM and RW). This may sound like a way to play pirated games, but [Deunan] makes it clear in his question and answer post that the firmware for his prototype is written to only play proper disc images and will probably not play the rips which are found in the darker recesses of the interwebs.
He’s been at this for quite a while. Here’s an earlier project he did that uses an FPGA board for the hardware.