What was the other use?
Modem and NIC were the only two purposes AFAIK. The Modems were pretty terrible, so if you didn't have a NIC for that slot, you'd likely pull it out to free up the plugged up serial port for use with a faster external modem.
Dial Up Networking is still, networking, even if it's only wide-area networking and not local area networking.
You got me there! l set up a sweet remote control system with one way VidCap over landline with Timbuktu Remote in the day.
It would be neat to find out that there was like a CSII sound card or a CSII SATA card.
IIRC someone made a 10/100 NIC for CSII, but that's probably the only thing that works? I doubt we'll ever find anything else. The NIC interface on CSII is a half baked PCI slot (flopped cake with mal aforethought?) that doesn't work AFAK. I took a look at the pinouts, comparing it to PCI in an attempt to hijack the PCI Slot ID for a third standard slot with a PC riser, but I think it was a dead end.
We're off topic here, but it would be great if someone would take a whack at a Gigabit NIC for CSII. That would be oh so cool, the TAM crowd would eat that right up. CSII should handle that I'd think, een if the machines can't manage wring the most out of it, it'd be one mean hack.