If it is the same Cumulus I recall, there should be an external port on the card that the cable attaches to. The other end of the cable should go to the floppy connector. It permits the addition of an external 5.25" floppy drive. Since all those cables hook the 5.25" drive to the primary controller, the 5.25" drive could be used as a boot (a: ) drive which really matters with some software. I think the Cumulus card only works with the Cumulus external drive. It would be unfortunate if the previous owner left out the external drive because it is basically useless without a Cumulus card.
Yes, that is a lot of cable but the card could be mounted in a full-size PC or XT chassis. An MCA variation of the card was also produced. Long cable is just right in towers.