Anyone use this or can explain it?

SP-1200 has a minimal midi spec so this device just improves performance with other outboard gear. My assumption is that the 1200 doesnt send a note off, so this box will handle that for you to equipment that requires it. The note off is just the same note repeated at 0 velocity to allow sequencing of synthesizers. I seem to remember these being pretty popular 5-10 years ago and a “must have” for folks using their SP with an S-950. The main use case would be if the SP-1200 was the centerpiece of your setup.