The MMC-88 is designed to offer MIDI control over the DA-88, if the SY-88 card is not installed in the machine. This will convert MMC to DTRS transport commands, and convert tape time (ABS) to MTC and SMPTE time code. This is ideal for giving sequencers, or basic digital consoles transport control over the DA-88, and synchronizing MIDI equipment.
If you already have a remote like the RC-848 or RC-828, you can connect that to the DTRS port on the rear panel. A switch on the MMC-88 determines whether control over the DTRS machine will come from the remote, or MMC.