ISA was used for EVERYTHING back then. Video cards, sound cards, network cards, modems, you name it. Hell, I think there was even a time when PC's would use the ISA bus for memory expansion... but that would have been before my time. A lot of that stuff is just built into the motherboard these days and we take it for granted, but I remember a time when I had to have a card for a floppy drive interface, another for a hard drive, and another just for serial and parallel ports. Back in those days, the fact that the SoundBlaster cards had a game port on them was a big selling point, because then you didn't need to use another ISA slot for a game port card!
One of the ISA slots in my DOS machine is being used for a special card that hooks to a separate box for reading Atari Portfolio memory cards. I also have a pair of ISA cards that hook to PCMCIA brakets that can mount in a 3.5" drive bay (Though I don't currently have them installed, since the cables between them are horrendous).
--Zero