Networked games, maybe, but probably not. You have to have the Duo on the desk next to the Mac to hook it up via the SCSI port. I'm not sure what you'd need a 56k modem for in this scenario. So say you had someone playing on the Mac, and you on the Duo one of these networked games. You'd already be sitting next to each other anyway, why not just whip out a second controller and both play the game on the Duo? No, I don't think this was the answer.
I think the idea was more along the lines of using the TurboDuo as a CD-ROM drive for the Mac. You have to remember that back in those days, CD-ROMs were not yet being shipped as standard equipment in new computers.
On the other hand, every Mac built from 1987 onward (until the late 90s) came with a SCSI port built-in.
TTi probably thought they could conceivably boost Duo unit sales if the Duo could also serve as an external CD-ROM drive for your home computer, as well.